I don't think it's compatible - http://www.oracle.com/technetwork/middleware/bi-enterprise-edition/bi-11gr1certmatrix-166168.xls
it normally works the other way round where you upgrade OBIEE and then the DB But you'll need OBIEE 12c if you have anOracle 12c DB.
It depends a bit on your OBIEE (there are tons of 11g versions) but it works fine with a DB 12c.
If it's a production or similar system then it's better if it is certified and for that you generally need to update your weblogic to 10.3.6 (from the base 10.3.5 provided with your OBIEE).
As said it works even without that upgrade but better to run a certified system to avoid potential issues and get support if needed by Oracle.
Also you don't mention which role the DB is playing: Is it holding the RCU or just the data you're analyzing?