Number of preferred stock and warrants for common stock issued.

No definition available.

Name:	open_StockAndWarrantsIssuedDuringPeriodSharesPreferredStockAndWarrants
Namespace Prefix:	open_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Exercise price per share or per unit of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21475-112644

+ Details

Name:	us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_ClassOfWarrantOrRightLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of securities into which the class of warrant or right may be converted. For example, but not limited to, 500,000 warrants may be converted into 1,000,000 shares.

+ References

+ Details

Name:	us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of warrants or rights outstanding.

+ References

No definition available.

+ Details

Name:	us-gaap_ClassOfWarrantOrRightOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Value of preferred stock and warrants for common stock issued.

+ References

No definition available.

+ Details

Name:	us-gaap_StockAndWarrantsIssuedDuringPeriodValuePreferredStockAndWarrants
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Value of outstanding derivative securities that permit the holder the right to purchase securities (usually equity) from the issuer at a specified price.

+ References

No definition available.

+ Details

Name:	us-gaap_WarrantsAndRightsOutstanding
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=open_PennyWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ClassOfWarrantOrRightAxis=open_SeriesDPreferredWarrantsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

WARRANTS (Details) - USD ($)	1 Months Ended	9 Months Ended
WARRANTS (Details) - USD ($)	Oct. 31, 2019	Sep. 30, 2020	Dec. 31, 2019	Dec. 31, 2018	Sep. 12, 2018	Jun. 12, 2018
ClassOfWarrantOrRightLineItems
Warrants to purchase shares						300,000
Class of warrants outstanding		225,076
Penny Warrants
ClassOfWarrantOrRightLineItems
Warrants to purchase shares					300,000
Exercise price of warrants (in dollars per share)					$ 0.01	$ 0.01
Warrants exercised		300,000
Warrants outstanding			$ 300,000
Series D preferred warrants
ClassOfWarrantOrRightLineItems
Warrants to purchase shares						5,320,038
Exercise price of warrants (in dollars per share)		$ 0.01	$ 0.01	$ 6.58		$ 6.58
Warrants exercised	5,320,038
Proceeds from exercise of warrants	$ 35,000,000	$ 3,000
Class of warrants outstanding			0
Warrants outstanding			$ 3,000,000