Series B Tranche Rights - Additional Information (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
Sep. 30, 2020	Mar. 31, 2020
Series B Tranche Rights [Member] \| Measurement Input Probability Of Achievement [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants and rights outstanding measurement		85.00%
Series B Tranche Rights [Member] \| Measurement Input, Discount Rate [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Warrants and rights outstanding measurement		0.20%
Series B Redeemable Convertible Preferred Stock [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of Series B preferred stock (Shares)	41,690,117
Share price	$ 0.8597
Series B Redeemable Convertible Preferred Stock [Member] \| Tranche Two [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Issuance of Series B preferred stock (Shares)	41,690,117
Share price	$ 0.8597
Proceeds from issuance of Series B preferred stock and tranche liability	$ 35.8

Temporary Equity Stock Issued During Period Shares New Issues

No definition available.

Name:	oncr_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues
Namespace Prefix:	oncr_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Warrants and rights outstanding measurement.

+ References

No definition available.

+ Details

Name:	oncr_WarrantsAndRightsOutstandingMeasurement
Namespace Prefix:	oncr_
Data Type:	num:percentItemType
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_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The cash inflow from issuance of callable preferred stock which is identified as being convertible to another type of financial security at the option of the issuer or the holder.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=121586364&loc=d3e3255-108585

+ Details

Name:	us-gaap_ProceedsFromIssuanceOfRedeemableConvertiblePreferredStock
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_FairValueByLiabilityClassAxis=oncr_SeriesBTrancheRightsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=oncr_MeasurementInputProbabilityOfAchievementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputDiscountRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementClassOfStockAxis=oncr_SeriesBRedeemableConvertiblePreferredStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	oncr_TrancheAxis=oncr_TrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: