So this dude just opened 8bb from mp and I had ATo in the BB around 45bb stack effective. I just folded but what should I do? Thanks.

Unless he is a maniac, folding is fine. With the right read jam and be happy to run it against, whatever he show up with.

When somebody opens 8bb from anywhere it is almost always JJ. Also possible are QQ and TT. Every now and then it is AA from somebody who just doesn't want to lose with AA. But we block AA here.

So its an easy fold for me.

Unless this is a very cheap/turbo tourney where this guy is always raising to 8bb. Then it is possible that a shove would be worth it.

What about 5bb? presumably a call?

We cant narrow his range down this much. Last time I faced an oversized open like this, I jammed AK for around 45 BB effective, and the opponent called with KJ. This was a 10$ online SnG, so not exactly high stakes, but not the lowest stakes online either. But I still find it ok to fold AT without a clear read. Even if we are against a hand like KJ, its not like, AT is a huge favourite.

Yeah with no other info I'd just fold. That's a bizarre bet that can be difficult to assign a range to, but I don't think any hands you dominate are going to be doing it.

Plus, being offsuit gives you fewer postflop options, and you're probably going to be in a tough spot when you flop top pair unless you just decide in advance you're going to go with it if you do. (The problem with that is that if you get it in on the flop and get called, you're probably crushed-- overpairs to your ten or aces with a better kicker. I would not expect to see A9 do this. Maybe 88 or 99 would do this and get it in on a ten-high flop, but that's a small percentage and a really exact set of possibilities you're hoping for.)

Not for me. 5bb is also likely to be JJ (and less so QQ/TT). Again unless the player is always opening 5bb I would fold. even if the player has AK its not a good spot for us. Aside from a KQJ, ATx, or TTx flop we are not going to know if we are ahead even if we hit a pair - though on a Axx flop with no J I would feel good about it. Still I think we make very little vs JJ/QQ/TT with Axx on the board OOP.

In all the tournaments I've played I only once saw a guy always open raise 5 bb's. I ended up losing to him with KK when I jammed with 18 bb's and he had to call with A9o.

So in general when somebody opens 5 bb's (or 8 bb's) they are doing it because they don't want anyone to call because of the nature of their hand. It is mostly JJ in my experience. I often say "Jacks?" when everybody folds and more often than not I get a sheepish grin/nod. It should be noted that I am definitely tapping the glass but it leads to some laughs and the players are not good to begin with so I doubt it will change their approach.