How SharePoint Online connect with Oracle HRMS without Azure

Question

"i am having a SharePoint 2010 application which was developed as 10% SharePoint for front end and 90% for Dotnet for backend. And we have a page load authorization system which get the user list from AD.And also we have using HRMS Oracle for some properties and using LINK Server we have done communication for SharePoint AD and Link server Via Oracle HRMS. But now we want to migrate the application to Sharepoint online but we don't have azure Sql.So how can we connect with HRMS." and how we will the approach for migration??