Will going for the international history Olympiad have an weightage in my application
Only if you do well.
If history is something you love, something that intrigues you, then the IHO(I’m not sure if this is what it’s actually called) will compliment that.
International competitions are always great on an application, especially Olympiads, even if it’s not one of the popular ones.
Go for it.
If you rank.