Hi,
Connection between the OP270 and S7-315 is set up in WinCCFlex, NOT on the OP. If it is selected Profibus in WinCCFlex then it'll use Profibus.
The only thing that can be changed using the 'S7-Transfer Settings' are bus speed, bus address and if OP270 is master on bus, it does not select which protocol to use.
Please state the problem you encounter a bit more precise.
Jeebs