"Off the hook" generally refers to not getting into trouble for something you probably should of. For instance: Teacher: "So, who studied for the exam we have today?" Students: ".... .. ..." Teacher: "Well, unfortunately the copier is broken. Looks like you're off the hook for today." This is a reference to fishing. Where it's certainly not good for the worm to be on the hook. Being off the hook means he lucks out and doesn't get eaten. There's also a more recent usage that is used by younger people, less standard and more popular culture. Saying something is "off the hook" means that it's really great or something I guess, as in "This party is off the hook!" (I feel like a child just typing that haha) which means it's a really good party.

"Get off the hook" means for instance that somebody got in trouble, but luckily the problem passed him by and everything was ok.