It appears that the only Oracle Client releases certified to run on Windows 2003 Server are 9.2 and above, so you should not install Oracle 7.3 or 8i as this is not supported. The certification matrix can be found here.
With respect to your question regarding OCI calls, review the Oracle Call Interface Programmer's Guide for either the Oracle 9.2 or 10g client, depending on which version you decide to install. The OCI documentation can be found on Oracle's Technology Network Web site. Click on Documentation --> Documentation Index and then choose the appropriate documentation for the release you have chosen to install.
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