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You can do that by passing the Opportunity Id into the Flow via the custom button URL as explained below:
https://developer.salesforce.com/docs/atlas.en-us.workbook.meta/workbook/flow_6_add_custom_button.htm/flow/Insert_Flow_API_Name?vOpportunityId={!Opportunity.Id}&retURL=/{!Opportunity.Id}
vOpportunityId: Flow variable for storing Opportunity Id
{!Opportunity.Id}: Merge field for Opportunity Id