Where are the Orings locate do nthe exhaust?

if you mean Gaskets...there is one at every intersection of the exhaust pipe~ particularly important on the manifold connection...if this leaks would make a whole lot of noise~

Its making noise but not real loud, the mechanic supposedly replace the ones on the manifold, where else would there be any?

And there are 2 0-rings, probably in front and back of the cat. It says 1 inlet and 1 outlet. I can't tell by the pictures I have to see for sure. If you have a leak you should be able to find it easy enough, Look at the flanges, they would be covered with black soot.