Thanks for the quick reply. Apparently I was mistaken when I said in my post that Arc SDE 9.3.0 didn't support Oracle 11G, looking further I found that SDE 9.3.0 does support for Oracle 11G R1 but only the 32 bit version, so if I install ArcSDE 9.3.0 for Oracle 11G
it should work without upgrading the ArcGIS Engine to 9.3.1. also, I was able to run one of my applications with ArcGIS Engine at 9.3.0, ArcSDE at 9.3.1 and Oracle 10G, I did have to upgrade my SDE geodatabase to bring it to the 9.3.1 level.
While a mixed installation of Engine 9.3.0 and SDE 9.3.1 seems to work at least under windows, it's probably not a good idea to install different versions of ArcGIS Engine and SDE.
Yeah, 9.3.0 support life has ended and 9.3.1 is about to end, that's a different issue all together, for now I live in the ArcGIS 9.3 world.
Thanks