It looks like the second public platform release for Oracle 11g is (surprise, surprise) Linux x86-64. Downloads are available on OTN.
As with previous Linux releases, 32-bit Oracle with a 32-bit OS and 64-bit Oracle with a 64-bit OS are supported, but 32-bit Oracle with a 64-bit OS is not.
Note also that sqlplus does not play well with SELinux under RHEL5; workarounds are to disable SELinux entirely, or to manually change the context of Oracle libraries to textrel_shlib_t. More details are in MetaLink note 454196.1 (login credentials required).