The compatibility matrix of SCM products says that ** 32 bit oracle client is required to connect to oracle11g 64 bit server.
As a general rule, the bit-level of the Oracle Client should always match the bit-level of the SCM Server - 64-bit Oracle Client with 64-bit SCM Server, and 32-bit Oracle Client with 32-bit SCM Server. On Unix and Linux this means you will always need the 32-bit Oracle Client, since the SCM Server is only offered in 32-bit format.
The only exception is when you are upgrading from Harvest 7.1 and you have to upgrade the database. The 64-bit SCM Server components (hdbsetup and convpw) can't be used to upgrade a Harvest 7.1 database, because part of the upgrade process involves converting all the passwords inside your Harvest database from the old "CACrypto" encryption format used with Harvest 7.1 to the new "ETPKI" encryption format used for SCM 12.1. The CACrypto libraries are needed for this conversion and they are only available in the 32-bit format. So, for purposes of converting your database you will need to install the 32-bit versions of the Oracle Client and SCM Server. Once the Harvest database upgrade is complete, you are free to install the 64-bit versions of Oracle Client and SCM Server and everything should work correctly from that point.