I know there are advantages to using the dealer, however o2 sensors are $170
each at Auto Zone and they are mode by Bosch, same folks who make the one in
that volvo box.

680 plus say 2 or 3 hrs time for diag and r&r (70/hr) and say a 25% parts
mark up is  well under 1100.  Volvos use a common fuel injection system and
any skilled techs who work on VW's or many Japanese cars have the skills and
equipment to dx a volvo too, to say nothing of BMW, Benz or Saab shops.

The independants also have no easy income from new customers, so they need
to make you happy first time every time.  In addition the dealer tends not
to see older and less well maintained cars then dealer techs making an indy
a better place for old car repair--IMHO.

Look for a place that a guy (or woman) owns where S/He is there very day
either wrenching or in the office.
Nice NAPA or certs from whatever place insure nohing, but a shop that has a
constant and changing flow of cars of a complexity of at theast a Volvo (new
VW at a minimum I would think, or loads of older rice burners) seems like a
good bet to me.

Talk to the people, see if they like what they do, if they see a dificult
job as a mystery to figure out, a game rather then a headache.

YMMV