UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 6-K

Report of Foreign Private Issuer

Pursuant to Rule 13a-16 or 15d-16

of the Securities Exchange Act of 1934

For the month of October 2015

Commission File Number: 1-15256

OI S.A.

(Exact Name as Specified in its Charter)

N/A

(Translation of registrant’s name into English)

Rua General Polidoro, No. 99, 5th floor/part – Botafogo

22280-001 Rio de Janeiro, RJ

Federative Republic of Brazil

(Address of principal executive offices)

(Indicate by check mark whether the registrant files or will file annual reports under cover of Form 20-F or Form 40-F.)

Form 20-F:  x            Form 40-F:  ¨

(Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(1)):

Yes:  ¨             No:  x

(Indicate by check mark if the registrant is submitting the Form 6-K in paper as permitted by Regulation S-T Rule 101(b)(7)):

Yes:  ¨             No:  x

(Indicate by check mark whether the registrant by furnishing the information contained in this Form, the Registrant is also thereby furnishing the information to the Commission pursuant to Rule 12g3-2(b) under the Securities Exchange Act of 1934.)

Yes:  ¨             No:  x

If “Yes” is marked, indicate below the file number assigned to the registrant in connection with Rule 12g3-2(b):

Oi S.A.

Corporate Taxpayers’ Registry (CNPJ/MF) No. 76.535.764/0001-43

Board of Trade (NIRE) No. 33.300.29520-8

Publicly-Held Company

NOTICE TO THE MARKET

On September 14, 2015, Oi S.A. (“Oi” or “Company”) received notice from the New York Stock Exchange, Inc. (the “NYSE”) that it was not in compliance with NYSE’s continued listing standard that requires that the average closing price of a company’s listed securities not fall below $1.00 per share for any consecutive 30 trading-day period.

Under NYSE rules, Oi has a period of six months from the receipt of the NYSE notice to regain compliance with the minimum share price requirement. During the interim period, Oi’s American Depositary Shares, each representing one Oi common share (“Common ADSs”), and Oi’s American Depositary Shares, each representing one Oi preferred share (“Preferred ADSs”), will continue to be listed and traded on the NYSE, subject to the Company’s compliance with other NYSE continued listing requirements.

Oi intends to amend the terms of its Common ADSs and Preferred ADSs to increase the number of common shares and preferred shares of the Company represented by the Common ADSs and Preferred ADSs, respectively, in order to regain compliance with the NYSE’s minimum share price requirement. Oi has not yet determined the applicable ratios of shares per ADS, but intends to select these ratios with the expectation that following these amendments, Oi will be in compliance with the NYSE’s minimum share price requirement for the foreseeable future. The amendments of the terms of the Common ADSs and Preferred ADSs must be approved by Oi’s board of directors and Brazilian Securities Commission (Comissão de Valores Mobiliários). Oi intends to implement this change in ratio prior to March 14, 2016.

Rio de Janeiro, October 13, 2015.

Oi S.A.

Flavio Nicolay Guimarães

Chief Financial Officer and Investor Relations Officer

Special Note Regarding Forward-Looking Statements:

This communication contains certain forward-looking statements. Statements that are not historical facts, including statements about our beliefs and expectations, business strategies, future synergies and cost savings, future costs and future liquidity are forward-looking statements. The words “will,” “may,” “should,” “could,” “anticipates,” “intends,” “believes,” “estimates,” “expects,” “plans,” “targets,” “goal” and similar expressions, as they relate to Oi, are intended to identify forward-looking statements and are subject to a number of risks and uncertainties. There is no guarantee that the expected events, tendencies or expected results will actually occur. Such statements reflect the current views of management of Oi and are subject to a number of risks and uncertainties. These statements are based on many assumptions and factors, including general economic and market conditions, industry conditions, corporate approvals, operational factors and other factors. Any changes in such assumptions or factors could cause actual results to differ materially from current expectations. All forward-looking statements attributable to us, or persons acting on our behalf, are expressly qualified in their entirety by the cautionary statements set forth in this paragraph. Undue reliance should not be placed on such statements. Forward-looking statements speak only as of the date they are made. Except as required under the Brazilian and the U.S. federal securities laws and the rules and regulations of the CVM, the SEC or of regulatory authorities in other applicable jurisdictions, we do not have any intention or obligation to update or to publicly announce the results of any revisions to any of the forward-looking statements to reflect actual results, future events or developments, changes in assumptions or changes in other factors affecting the forward-looking statements. You are advised, however, to consult any further disclosures Oi makes on related subjects in reports and communications Oi files with the CVM and the SEC.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.

Date: October 14, 2015

OI S.A.
By:	/s/ Flavio Nicolay Guimarães
	Name:	Flavio Nicolay Guimarães
	Title:	Chief Financial Officer and Investor Relations Officer