Hi!
A 10x5 prop is prefect for the OS .26. Anything smaller is not going to work as good. You would expect the engine to rew 10000 rpm with a APC 10x5 or Master Airscreew "scimitar" 10x5 when using 5% nitro fuel. This is at sea level.
If your OS .26 is rewing only 9000rpm with a 10x5 prop either the cam is set wrong or the engine is worn out.