I notice that a lot of people at my club use 10% nitro fuel. Is there any advantage to using 15 or 20% mixes in my OS 120 AX?

The 120AX seems to do best overall with the 10%

I run a 120AX. The biggest difference you will see in using higher % nitro is more fuel consumption. You have to open the needle valve to let in more fuel as higher nitro burns in less oxygen. I find no need to use more than 10%; I think it's a waste of money and fuel. There is a slight increase in power with more nitro, but it's not very noticeable.

I think 20% you start to come close to timing issues and predetonation. YS engines use at least 20% nitro, but they are a different animal.

I have run mine on 5% to 15% with no problems.