Is putting in a new catalytic converter and/or O2 sensor a do it yourself job if you know what your doing?

Depends.

Cat is bolted  on my old Chevy. Still might take special tool.

O2 sensors are a snap.

Even If welded-on, a carbide wheel at Harbor Freight is cheap. Put it on  a drill. A tailpipe expander, too.

 Extension pipes and  clamps at any auto parts place. 

Best test.  If you need to ask, a CAT is no DIY.  Just leave the dam thing alone. A CAT can cost you more than the car is worth.

An O2 sensor is fairly easy for most people. The catalytic convertor is more than most people are equipped to handle. If you have to ask, you have ZERO business messing with the catalytic converter.

No. Especially if you don't have access to a torch as back up when messing around with exhaust parts. 

Most important item to investigate is whether or not the catalytic converter is welded to the exhaust or bolted to the exhaust. That's the $2,000 question. Bolted can be easier if you can get those old corroded bolts out with or without a torch. (Propane or MAPP gas torch for the DIY guy) Might have stainless bolts, might have grade 5 or grade 8 bolts. If it is welded to the exhaust (a good number of cars have stainless steel exhaust from the exhaust manifold to the tailpipe) which is very difficult to cut with a saw and usually has to be cut with a oxy/acetylene  torch.

O2 sensor is a walk in the park. 

You know anyone that has to ask this certainly doesn't have a clue what they are doing or it would be done already instead of asking strangers dumb questions.

You put the car on a lift or jack stands and see if you can do it.  A cat converter is usually welded, not a DIY evolution.

O2 sensors are a breeze.  Your cat may be another matter.

You Tube could help you there.  Cats cost so they are welded on(less likely to be stolen) Muffler clamps won't do.  If you need a cat there is a good chance the whole exhaust pipe needs replacing...due to rust.  While I can do it, I won't and let a muffler shop do it.

If you knew what you were doing you wouldn't be asking here. Cats are usually welded on at one end.