User had Toad 9.1 installed then downgraded back to Toad 7.5. Now Toad 7.5 gets error message "OCI version 10.2.0.1.0 is not supported" when using the Schema Browser.
Toad for Oracle 7.5 does not officially support 10g databases or clients. User may have upgraded or changed the client during testing of Toad 9.1.0 and so when user reverted back to 7.5, it no longer worked. Workaround is to remain on the newest version of Toad.