I bought a used '95 Turbo Wagon that had 3 sources of oil leaks - all found and fixed myself. There's two likely causes of the leak you described.

1. The turbo return line (as someone else had responded). The bottom of the return line goes right in above the oil pan. The original o-ring shrinks and the leak starts. Volvo has upgraded the material from a black rubber to a heat resistant green and it's a cheap part. To replace it, you'll have to take the two bolts loose from the turbo housing and the one bolt that connects the o-ring end. Get a turbo oil line gasket also (cheap) and change that out at the same time.

2. Oil dipstick tube. The o-ring at the bottom of the oil dip stick tube is like the one for the turbo return line - same fix. Also the dipstick itself has an o-ring that should be replaced.

If these don't work post again and I'll get into my other leak repairs...

Good luck - Scott '95 855t /'88 745gle

If it is a turbo the oil return line is common problem (and an
easy fix). See:

http://volvospeed.com/Repair/turborepairline.htm

I don't recall too many (or any) posts here about leaking oil
pans -- leak is useually up higher.

If your car is not a turbo, then you may need your flame trap cleaned/replaced. This only costs about $3.00 for parts and 10 minutes for labor. I had a similar leak on my 1990 740 GLE 16v and once I replaced the flame trap the leak stopped immediately. One mechanic said it was a cam/crank seal leak $500 plus, another said it was a main seal leak $1,000 plus, but it was just a clogged flame trap $3.00...easy fix. This is only a suggestion of a possibile cause.