MySQL and PostgreSQL is officially supported DBs for community versions of Alfresco. Oracle is supported for Enterprise version. But you can still do it if you want to do it. Look at this repo if its helpful: https://github.com/krumboeck/oracle-alfresco
Some comments about this. Although connecting to an Oracle database is technically possible in Alfresco Community, the integrator should consider that (s)he is probably losing all the database testing done by Community users on the used (and supported) databases. If some DB issue appears (s)he is basically alone. Besides, the integrator is not probably considering that in some moment you may have to upgrade the schema, for example when upgrading to the next GA and new problems may arise. Customers sometimes say they have Oracle support, but Oracle support in these situations will say it is a product related issue, it is a bug of the product or something similar.
The same applies if you are not using the correct driver or the correct database version in Enterprise Edition, you are ignoring the database testing done by Alfresco team and in this case in addition, losing full Alfresco support.
In addtition I should add that though it is technically quite possible to use Oracle for Alfresco Community (I tried it myself, it works) you should think "What for".
The structure of Alfresco database is very plain, it consists of tables only, no program objects like functions, procedures.
I suppose , the indexes on the tables in Alfresco database are optimised for all regular queries.
Using Oracle will not add significant performance.
All the performance gain from Oracle may be obtained just by transferring PostgreSQL server to another physical host different from Tomcat server host with sufficient disk storage performance.
Another argument in such cases is presence of Oracle DBA in a company and absence of PostgreSQL DBA.
But as I wrote, the structure of Alfresco database is plain, according to my experience PostgreSQL database with size up to 1GB and I suppose larger does not require DBA.
And finally backing up Alfresco instance includes backup of of database and of repository (and indexes but not necessary) and what is important, it should be done at the same moment, DB and repository should be completely synchronous. While for Alfresco with PostgreSQL you may find ready to use scripts, it is quite a problem to make synchronous backup in case of Oracle.