I found 2 18mm threaded nuts out in the garage, and the o2 sensors fit fine in there, is this the same thing as a o2 bung you would buy , i mean i could weld the nut on and it would serve the same perpouse! Still screw in and screw out!

 

I don,t see a reason that would not work.
Just make sure the O2 sensor is far enough in the pipe, it needs to stick out into the exhaust stream a bit, know what I mean?
make sure the welds seal the nut or you will have an exhaust leak, O2 bungs are round and weld in well. If you pay attention to the details it should work fine.
You can pick up O2 bungs at Midas or Mieneke (sp?) for cheap though.