Migration of SQL Server to Oracle Database
Global temporary tables in Oracle are very different, as they have a permanent logical definition and transient storage, so practically the reverse of your situation.

I would look at trying to avoid temporary tables at all, in favour of Common Table Expressions (WITH clauses). They are very powerful stuff and can transiently use temporary disk space if it aids performance.

Otherwise you'll have to manually code unique table names etc -- it'll be unpleasant to support in future.