I haven't done this yet but there are universal O2 sensors available from auto parts stores that you just splice into the existing wiring. I would get a Bosch sensor, I think they are the original supplier.

Steps:

1. remove old sensor and disconnect from wire harness.

2. cut and splice (crip connector and solder is a nearly permanent connection) new sensor and old wire harness connector (the part from the old sensor).

3. heat shrink around the splice.

4. install new sensor and connect to harness

I am sure there are sensors from production vehicles that also fit. Then you get the vehicle side harness/o2 sensor from a junk yard and splice that into the Volvo's O2 side harness connector and you have a plug-n-play application.