Look for vacumn leaks at the manifold elbows first. Check:

http://www.volvospeed.com/Repair/vacuum_lines.htm

If they're ok and you need an O2 sensor you can get a Bosch direct replacement from fcpgroton.com for about $120. There's a universal available cheaper but you have to splice the wires.

Check the wiring for burns, dirty plug (the connector plugs into the wiring harness at a bracket on the top rear of the transmission) etc.

You'll find the sensor screwed into the exhaust pipe between the exhaust manifold and the catalytic converter. It's about 2" long and has wires coming out of the end. No special wrench required. If you have a turbo the connector might be tough to reach but try to get at it from the top and bottom.

The new sensor will have anti-seize compound on the threads so just screw it in and tighten.

Reset the codes and drive. The CEL might stay on for a while but will go off in a day or so.

If you need a MAF (also known as the AMM, Air Mass Meter) shop around through the advertisers here for the best price. As you've discovered, that's an easy swap.

Good luck,
--
Erwin in Memphis, '88 745t 190,000 miles, '95 855t 92,000 miles