Cash, Cash Equivalents and Short-term Investments - Summary (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 53,637	$ 67,230	$ 114,448
Gross Unrealized Gains	0	0	3
Gross Unrealized Losses	(9)	(27)	(63)
Fair Value	53,628	67,203	114,388
Cash and money market funds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,601	15,703	43,930
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	24,601	15,703	43,930
Corporate debt securities: Due in one year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		38,053	48,492
Gross Unrealized Gains		0	3
Gross Unrealized Losses		(21)	(29)
Fair Value		38,032	48,466
U.S. Government Debt Securities Due In One Year Or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost		13,474	14,013
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(6)	(16)
Fair Value		13,468	13,997
U.S. Government Debt Securities Due In Two Years Or Less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost			8,013
Gross Unrealized Gains			0
Gross Unrealized Losses			(18)
Fair Value			7,995
Cash and cash equivalents
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,601	15,703	43,930
Gross Unrealized Gains	0	0	0
Gross Unrealized Losses	0	0	0
Fair Value	24,601	15,703	43,930
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	29,036	51,527	70,518
Gross Unrealized Gains	0	0	3
Gross Unrealized Losses	(9)	(27)	(63)
Fair Value	$ 29,027	$ 51,500	$ 70,458

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 5
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562

+ Details

Name:	us-gaap_AvailableForSaleSecurities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

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

- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.2)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_AvailableForSaleSecuritiesAmortizedCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_InvestmentTypeAxis=ovas_CashAndCashEquivalentsAndMoneyMarketFundsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ovas_CorporateDebtSecuritiesDueInOneYearOrLessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ovas_U.S.GovernmentDebtSecuritiesDueInOneYearOrLessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=ovas_U.S.GovernmentDebtSecuritiesDueInTwoYearsOrLessMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_CashAndCashEquivalentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_InvestmentTypeAxis=us-gaap_ShortTermInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: