You will find OCCI 10g as part of 10g client installation. But that does not work with VS2010. You need to download the 220.127.116.11 OCCI libraries that are compatible with VS2010 and use them with 18.104.22.168 client software.
You can use that above (22.214.171.124 client) to connect to a 10.2 database.
\connectivity.cpp(20) : error C2872: 'Environment' : ambiguous symbol
1> could be 'C:\Program Files\Microsoft Visual Studio 9.0\VC\include\occiControl.h(732) : oracle::occi::Environment'
1> or 'c:\windows\microsoft.net\framework\v2.0.50727\mscorlib.dll : System::Environment'
Did you check if OCCI dlls in 10g XE are built using VS2008. If not you have to use the same version of VS used in building the OCCI dlls.
You can do dumpbin to know the dependency and the version from it.