How do I create an Oracle 11g user with read access to all tables/schemas?
You can grant SELECT ANY TABLE, however, that is seen by some as "excessive" privileges, and should be questioned.

On the plus side, Oracle 11 keeps key system views hidden even from that grant .. so it's not nearly as bad as it once was ;)

It would be best to build ROLEs and grant the SELECTs to the required tables to the ROLEs ... then grant the ROLE to your users as needed.

If you haven't yet, you should probably read over this guy:

https://docs.oracle.com/cd/B28359_01/network.111/b28531.pdf