Yes it can be he or she, that depends on the context. It can be about someone in polics for example.

O can mean he, she, it or even that.

The context of what was said or written before should guide you to the correct choice.

Also güç has 2 different meanings, power/strength or hard/difficult.

You would have to mention what was mentioned prior to this sentence to get an accurate meaning.

"Güç" here can´t mean "difficult", because it´s been modified by "büyük". "Büyük güç" would only mean "big power".

Also, since "O" is capitilized, this pronoun must substitute for God; because we often capitilize the pronoun "O" when we talk about God.

So the sentence would translate to: "He´s (God) the biggest power." 

Hope it was helpful,

-Jansay

So with the capital O it would not be he or she? 

Thanks for explaining about the work büyük 

It depends on the subject in the sentence. It´s not complicated as you think. 

O = S/he, it.