As the others have said, it's NEVER the turbo unit itself that's somehow the cause of your oil leak. It's a simple device yet obviously often misdiagnosed by those unfamiliar with turbochargers.
Besides the drain tube o-ring listed below, another common source of seapage seen from the bottom of the turbo itself is coming from the intake air tube as there's always some oil mist in all turbo air hoses. For that, simply tighten the large clamp on that hose at the turbo itself (hose between turbo and MAF meter).

DO NOT remove the turbo...it's undoubtedly a $2 oil return seal. Volvospeeds nifty illustrated repair procedure can be found at http://volvospeed.com/Repair/turborepairline.htm

You probably need more persistence than skill for this one, but it won't cost you a lot of money.
--
(98 S70 T5SE misc mods - mostly lighting - and red calipers) (92 940GLE)

Most 850's will leak oil. This leaking will come from to main places.

1)Turbo oil return line
2)rear main seal

The turbo return line is cheap to do but the rear main seal requires removal of the gearbox.

go to volvospeed.com bay 13 which details the turbo return line job.

Most common is a leaky oil return line. Leaky turbo seal is uncommon.
Simple repair for the oil return line and you should investigate
this as a cause before touching the turbo.

See http://volvospeed.com/Repair/turborepairline.htm
for repair instructions.