You can and I will also suggest you to try Mamaearth Easy Tummy Roll On.

If you have already introduced these, then it’s okay to mix oats and apple

Yes I add puree of apple in oats perfectly fine

I have introduced sometime back I have introduced Apple and other things individually and he is fine with all these.. And I feed him apple rice too..np with that.. But one of my friend told, fruits ll get digested soon, so don't mix those too.. that ll affect the digestion of the baby.

Firstly introduce one thing at a time. Even after giving 3 days in a row, baby doesn’t show any allergy or indigestion, then you can introduce other items. You can mix and give, but what matters is when did you start solids for your lill one? If baby has got adjusted with solid food then there won’t be digestion problems.