Actually, Moodle HQ are able to test issues on Oracle if necessary, but they only seem to do so if it is an Oracle-specific bug, or there is a good reason to believe that the change might break on one database, but not another.
Anyway, Howard has a point. There are two problems with Oracle:
1. Of all the supported database, Oracle is the one that is furthest from the SQL standard (the worst problem is its inability to distinguish NULL (unknown value) from "" (empty string)). Hence, to make it work,
2. The protocol that Oracle uses to transfer data from the database to Moodle does not perform well for web applications which use a lot of CLOB columns, which exactly describes Moodle. For forum threads about this:
- https://moodle.org/mod/forum/discuss.php?d=65488#p301613
- https://moodle.org/mod/forum/discuss.php?d=143376#p653822
- https://moodle.org/mod/forum/discuss.php?d=87509#p389413
Both of those problems are pretty much unfixable. Moodle has done what work-around is can, but they just add to the slowness and bugginess.
So, really the only reason to use Oracle is if institutional politics really forces you to. If you can use Postgres, which is somewhat similar to Oracle, but free, highly SQL standards compliant, and fast.