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v3.25.2

Related Parties - Additional Information (Details) - USD ($) $ in Millions | 12 Months Ended
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Mar. 31, 2025 | Mar. 31, 2023
Sumitomo Corporation |  |
Disclosure of transactions between related parties [line items] |  |
Cash consideration |  | $ 12.8
Related Parties |  |
Disclosure of transactions between related parties [line items] |  |
Cash consideration |  | 11.6
Additional cash payment to associates |  | $ 0.8
Bottom of Range |  |
Disclosure of transactions between related parties [line items] |  |
Amount receivables, outstanding balances settlement period | 30 days |
Amounts payable, outstanding balances settlement period | 30 days |
Top of Range |  |
Disclosure of transactions between related parties [line items] |  |
Amount receivables, outstanding balances settlement period | 60 days |
Amounts payable, outstanding balances settlement period | 60 days |

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- Definition Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table. + References No definition available. + Details
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Namespace Prefix: | ifrs-full_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Additional cash payment to associates. + References No definition available. + Details
Name: | omse_AdditionalCashPaymentToAssociates
Namespace Prefix: | omse_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Related party transaction cash consideration. + References No definition available. + Details
Name: | omse_RelatedPartyTransactionCashConsideration
Namespace Prefix: | omse_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Related party transactions amount payable settlement period. + References No definition available. + Details
Name: | omse_RelatedPartyTransactionsAmountPayableSettlementPeriod
Namespace Prefix: | omse_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Definition Related party transactions amount receivable settlement period. + References No definition available. + Details
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Namespace Prefix: | omse_
Data Type: | xbrli:durationItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_CategoriesOfRelatedPartiesAxis=ifrs-full_RelatedPartiesMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_RangeAxis=ifrs-full_BottomOfRangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | ifrs-full_RangeAxis=ifrs-full_TopOfRangeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |