When I was being evaluated for my transplants I had to make sure that my O2 stayed above 90%. If it dropped below that, that's when I had to wear my O2. Of course, it got to where it was always below 90 so I was wearing it all the time....but I think that going below 90% is what most doctors consider low.
As for HR, mine has always been high. It has only dropped below the 100's since the Tx.