If IEI is 64-bit, then the Oracle Client version will need to be 64-bit as well.
For Domino/IEI 9.0.1 64-bit on Windows 2008 64-bit, we specifically support the Oracle 11G native client. Also, if you have the opportunity to install the Full Client instead of the Instant Client, I'd recommend it.
For reference, see the 9.0.1 release notes:
https://www.ibm.com/developerworks/lotus/documentation/lei/
http://publibfp.dhe.ibm.com/epubs/pdf/i1329171.pdf