Resolution:
OS support depends on both the adapter version as well as vendor support. TIBCO Adapter for Siebel can only support the platforms that Siebel Server supports for any specific version.

For Windows 2008, Siebel 8.0.0.3 and higher is required.

For HPUX Itanium 11.31 v3, Siebel platform guide does not state support for this platform in their platform guide. The most current version Siebel support is HPUX Itanium 11iv2.

For Siebel Adapter 5.3.1 for Siebel Application 7.5.3, the following OSes are supported:

* Windows 2000 on x86
* HPUX 11.11 (11i) on PA-RISC
* AIX 5.2 on POWER
* Solaris 8 on SPARC

References
Please refer to adapter readme.txt and following LBNs for a list of other OSes supported:
LBN1-9X0Z8V, LBN1-91LL39, LBN1-8QMZ6T, LBN1-9K0F7Z