I would say both ADHD and OCD have issues with not having control of what takes the center of attention. For OCD you get stuck on whatever compulsion and can't move past until it's resolved. ADHD has issues with being stuck on arbitrary things that capture their attention and can't reassign their focus at will even for important matters.

They don't really seem that different in terms of the consequences they have on someone's ability to function.

That's a very informal opinion though. Not a doctor and all that.