What is the worst that can happen with bad 02 sensors on a 1999 Volvo S80?

First make sure it is bad 02s , the 02 code isn't always just the 02 gone bad. Could be wiring problem or engine problems, injectors ,spark plugs, catalysts converter. The 02 system is part of the fuel control systems,cause over fueling to running lean mixture which can cause catastrophic engine failure to plugged up catalyst converter which can also cause engine failure. It's a pay now or pay lot more later