We see Ocelot giving some assistance to Big Boss to help find Kaz to reunite his army. But Big Boss was one of the founding members of the patriots, and he knew which included zero and of course Ocelot. The patriots (Zero) were the ones who attacked Mother Base twice and eventually destroyed it (Not PW spoiler, happens in MGSV) so why is Ocelot helping Big Boss, and why is Big Boss trusting ocelot? Edit: Never mind, I got the answer. Ocelot wa fighting for Big Boss the whole time, since he idolised him.