<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>107 - Disclosure - ORGANIZATION AND BASIS OF PRESENTATION</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><CurrencyCode /><FootnoteIndexer /><hasSegments>false</hasSegments><hasScenarios>false</hasScenarios><MCU><KeyName /><CurrencySymbol /><contextRef><ContextID>eol_PE3458----1310-Q0007_STD_181_20130630_0</ContextID><EntitySchema>http://www.sec.gov/CIK</EntitySchema><EntityValue>0000873303</EntityValue><PeriodDisplayName /><PeriodType>duration</PeriodType><PeriodStartDate>2013-01-01T00:00:00</PeriodStartDate><PeriodEndDate>2013-06-30T00:00:00</PeriodEndDate><Segments /><Scenarios /></contextRef><UPS /><CurrencyCode /><OriginalCurrencyCode /></MCU><CurrencySymbol /><Labels><Label Key="CalendarSupplement" Id="0" Label="6 Months Ended" /><Label Key="Calendar" Id="1" Label="Jun. 30, 2013" /></Labels></Column></Columns><Rows><Row FlagID="0"><Id>1</Id><IsAbstractGroupTitle>true</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>1</Level><ElementName>us-gaap_AccountingPoliciesAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>Accounting Policies [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>terseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="eol_PE3458----1310-Q0007_STD_181_20130630_0" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div&gt;
&lt;p style="MARGIN-TOP: 12px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;1. ORGANIZATION AND
BASIS OF PRESENTATION&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;!-- xbrl,body --&gt;
&lt;p style="MARGIN-TOP: 6px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;&lt;i&gt;Business&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Sarepta
Therapeutics, Inc. and its wholly-owned subsidiaries
(&amp;#x201C;Sarepta&amp;#x201D; or the &amp;#x201C;Company&amp;#x201D;) is a
biopharmaceutical company focused on the discovery and development
of unique RNA-based therapeutics for the treatment of rare and
infectious diseases. Applying the Company&amp;#x2019;s proprietary
platform technologies, the Company is able to target a broad range
of diseases and disorders through distinct RNA-based mechanisms of
action. The Company is focused on advancing the development of its
Duchenne muscular dystrophy drug candidates, including its lead
product candidate, eteplirsen, for which the Company is currently
conducting an ongoing open label extension study following
completion of its initial Phase IIb clinical trials. The Company is
also focused on developing therapeutics for the treatment of
infectious diseases, including its lead infectious disease program
aimed at the development of a drug candidate for the Marburg
hemorrhagic fever virus for which the Company has historically
received significant financial support from U.S. government
research contracts.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The
accompanying unaudited condensed consolidated financial statements
reflect the accounts of Sarepta and its consolidated subsidiaries.
The accompanying unaudited condensed consolidated balance sheet
data as of December&amp;#xA0;31, 2012 was derived from audited
financial statements not included in this report. The accompanying
unaudited condensed consolidated financial statements were prepared
in conformity with accounting principles generally accepted in the
United States of America (GAAP) and the rules&amp;#xA0;and regulations
of the U.S. Securities and Exchange Commission (SEC) pertaining to
interim financial statements. Accordingly, they do not include all
of the information and footnotes required by GAAP for complete
financial statements. The accompanying unaudited condensed
consolidated financial statements should be read in conjunction
with the financial statements and the notes thereto included in the
Company&amp;#x2019;s annual report on Form&amp;#xA0;10-K for the year ended
December&amp;#xA0;31, 2012. The results of operations for the interim
periods presented are not necessarily indicative of the results to
be expected for the full year.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Management has
determined that the Company operates in one segment: the
development of pharmaceutical products on its own behalf or in
collaboration with others.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;Since its
inception in 1980, the Company has incurred losses of $492.4
million, substantially all of which resulted from expenditures
related to research and development, general and administrative
charges and losses on change in warrant valuation partially offset
by revenue generated from research contracts with and grants
primarily from the U.S. Department of Defense (DoD). As of
June&amp;#xA0;30, 2013, the Company has completed all of its contracts
with the DoD except for the July 2010 contract and the August 2012
contract for the development of therapeutics against the Marburg
virus. The current period of performance for the August 2012
contract is scheduled to conclude in the second half of 2013
subject to additional extensions that may be agreed upon by the
Company and the DoD. In November 2012, the Company also entered
into an agreement with the European Commission (EC) Health
Innovation for development and study related activities for a
Duchenne muscular dystrophy (DMD) therapeutic for which minimal
revenues have been earned to date. The Company has not generated
any material revenue from product sales to date, and there can be
no assurance that revenues from product sales will be achieved.
Moreover, even if the Company does achieve revenue from product
sales, the Company is likely to continue to incur operating losses
in the near term.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;As of
June&amp;#xA0;30, 2013, we had $164.0 million of cash equivalents and
invested cash, comprised of $156.2 million of cash and cash
equivalents and $7.8 million of restricted investments, which the
Company believes, taking into consideration our current stock price
and outstanding warrants, is sufficient to fund our current
operational plan for the next twelve months. Should the
Company&amp;#x2019;s funding from the DoD cease or be delayed, the
Company would likely curtail certain of its infectious disease
research and development efforts unless additional funding was
obtained. The Company is also likely to pursue additional cash
resources through public or private financings, including the $37.9
million raised in our at the market offering described in note 6,
seeking additional government contracts, and from establishing
collaborations or licensing its technology to other
companies.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 18px; MARGIN-BOTTOM: 0px; FONT-SIZE: 1px"&gt;
&amp;#xA0;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 0px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;&lt;i&gt;Estimates and
Uncertainties&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The preparation
of financial statements in conformity with GAAP requires management
to make estimates and assumptions that affect the reported amounts
of assets and liabilities and the disclosure of contingent assets
and liabilities at the date of the financial statements and the
reported amounts of revenue and expenses during the reporting
period. Actual results could differ from those
estimates.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 18px; MARGIN-BOTTOM: 0px"&gt;&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;&lt;b&gt;&lt;i&gt;Commitments and
Contingencies&lt;/i&gt;&lt;/b&gt;&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 6px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;The Company is
not a party to any material legal proceedings with respect to
itself, its subsidiaries, or any of its material properties as of
June&amp;#xA0;30, 2013. In the normal course of business, the Company
may from time to time be named as a party to various legal claims,
actions and complaints, including matters involving employment,
intellectual property, and effects from the use
of&amp;#xA0;therapeutics&amp;#xA0;utilizing its technology, professional
services or others. It is impossible to predict whether any
resulting liability would have a material adverse effect on the
Company&amp;#x2019;s financial position, results of operations or cash
flows.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;In February
2013, the Company issued two letters of credit totaling $7.3
million to a contract manufacturing vendor in connection with
certain manufacturing agreements. To meet the requirement of the
letters of credit, the Company purchased $7.3 million in
certificates of deposit with April 2014 maturity dates in February
2013. The Company has recorded this $7.3 million as restricted
investments in the condensed consolidated balance sheet as of
June&amp;#xA0;30, 2013.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;In April 2013,
the Company and the University of Western Australia (UWA) entered
into an agreement under which an existing exclusive license
agreement between the Company and UWA was amended and restated.
Under the terms of this agreement, UWA granted the Company an
exclusive license to certain UWA intellectual property rights in
exchange for up to $7.1 million in upfront and development
milestone payments. During the three and six months ended June 30,
2013, the Company recognized $1.0 million relating to certain
upfront payments required under the agreement within research and
development in the condensed consolidated statement of
operations.&lt;/font&gt;&lt;/p&gt;
&lt;p style="MARGIN-TOP: 12px; TEXT-INDENT: 4%; MARGIN-BOTTOM: 0px"&gt;
&lt;font style="FONT-FAMILY: Times New Roman" size="2"&gt;In June 2013,
the Company entered into a lease agreement for its Cambridge
location. The agreement calls for a security deposit in the form of
a letter of credit totaling $0.6 million. The Company purchased a
certificate of deposit to meet the requirement. The initial term of
the lease agreement is for seven years with an average base rent of
approximately $2.4 million per year.&lt;/font&gt;&lt;/p&gt;
&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for the organization, consolidation and basis of presentation of financial statements disclosure, and significant accounting policies of the reporting entity. May be provided in more than one note to the financial statements, as long as users are provided with an understanding of (1) the significant judgments and assumptions made by an enterprise in determining whether it must consolidate a VIE and/or disclose information about its involvement with a VIE, (2) the nature of restrictions on a consolidated VIE's assets reported by an enterprise in its statement of financial position, including the carrying amounts of such assets, (3) the nature of, and changes in, the risks associated with an enterprise's involvement with the VIE, and (4) how an enterprise's involvement with the VIE affects the enterprise's financial position, financial performance, and cash flows.  Describes procedure if disclosures are provided in more than one note to the financial statements.</ElementDefenition><ElementReferences>Reference 1: http://www.xbrl.org/2003/role/presentationRef

 -Publisher FASB

 -Name Accounting Standards Codification

 -Topic 275

 -SubTopic 10

 -Section 50

 -Paragraph 2

 -URI http://asc.fasb.org/extlink&amp;oid=6927468&amp;loc=d3e6003-108592



</ElementReferences><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>ORGANIZATION AND BASIS OF PRESENTATION</Label></Row></Rows><Footnotes /><IsEquityReport>false</IsEquityReport><ReportName>ORGANIZATION AND BASIS OF PRESENTATION</ReportName><MonetaryRoundingLevel>UnKnown</MonetaryRoundingLevel><SharesRoundingLevel>UnKnown</SharesRoundingLevel><PerShareRoundingLevel>UnKnown</PerShareRoundingLevel><ExchangeRateRoundingLevel>UnKnown</ExchangeRateRoundingLevel><HasCustomUnits>true</HasCustomUnits><IsEmbedReport>false</IsEmbedReport><IsMultiCurrency>false</IsMultiCurrency><ReportType>Sheet</ReportType><RoleURI>http://www.sareptatherapeutics.com/taxonomy/role/NotesToFinancialStatementsOrganizationConsolidationAndPresentationOfFinancialStatementsDisclosureAndSignificantAccountingPoliciesTextBlock</RoleURI><NumberOfCols>1</NumberOfCols><NumberOfRows>2</NumberOfRows></InstanceReport>
