Hi Mark, here goes my 2 cents on the subject ...

First of all, the AgilePlatform will take care of all your worries in the entity definition and when using simple queries (your application will behave seamlessly with both RDBMS). This means that you only have to worry about Oracle specific issues when writing advanced queries.

That being said, here goes some additional hints:
. Don't forget to set your eSpace DBMS attribute to "Oracle"
. Check https://www.outsystems.com/help/servicestudio/4.2/using_data/oracle_compatibility_issues.htm to learn about some of the platform "goodies" to minimize compatibility issues with Oracle
. Use simple queries whenever possible
. The built-in functions are not the same. Check https://www.techonthenet.com/oracle/functions/ for a list of Oracle's built-in functions
. The new "count" property of the simple queries (since 4.2) will provide you the same result as a COUNT query, even if you specified "Max. Records" (no need to perform an advanced query just to do a count)

Hope this helps, cheers.