Can I use the “Database Link” tool in Apex to connect to MS SQL Server? Or just other Oracle DBs?
This is what the DBLink creation form looks like in Apex (red star indicates required field):

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DB Links are for other oracle databases only. Due to your situation (sql server and oracle integration), maybe you should think the other way around: create a linked server on sql server to connect to oracle and create a procedure there to send the data to oracle.