I think you've got multiple problems. ABS problem is not related to recent work. Vehicle speed signal is a give away. I'd suspect that the module has crapped out. This will also effect the way the transmission shifts, as it gets info from the ABS module and its knowledge of the vehicle speed and adjusts shifts accordingly.

Oxygen sensor code is a real bummer on this model. I'm not too familiar with "PO" codes, more familiar with Volvo specific codes. If the sensor is indeed dead, you may be in the market for ALL FOUR oxygen sensors. They work in concert and, more often than not, when only one is replaced, the other three soon start to post up codes. Better (relative term) to do all at once. Expect to pay over $1000 for correct sensors installed.

Good move to sell. Honestly, I wouldn't wish a 1999 twin turbo S80 with 200K on my worst enemy. And sitting around doesn't help, either. Good luck.