Is there any website where you can write oracle sql code, maybe even build a small database to work on, run it and see an output? I've tried searching online but I couldn't really find anything besides W3Schools website.
As of January 2024, SQL Fiddle has been completely redesigned/rebuilt and no longer supports Oracle. Previous fiddles included in (many) questions and answers are no longer available.
See this answer for alternatives.
I'll leave this answer here as a signpost for now.
SQL Fiddle will let you build schema objects and run queries against them. It's great for trying things out. It has become quite common for answers here to include a link to a Fiddle to demonstrate a code sample (including quite a few of mine, apparently).
It's free, but welcomes donations.