I note there are three o-rings used on the B234F oil separator box rather then just the one o-ring used on the B230F/FT oil separators. It will be best to replace all three to ensure a tight seal, definitely replace any that are no longer plump, soft and pliable.

Referring to your parts schematic they are p/n's 949659 (34mm i.d.), 968599 (13mm i.d.), and 949656 (19mm i.d.)

All appear to be currently available, even in the aftermarket, and some may well be in local dealer stock. In addition to PCV oil separator boxes, you will find some of those listed for other Volvo applications, such as distributor shaft seals, power steering seal, manual trans shift lever seal, ZF power steering rack seal and trans speedometer sensor seal.

As much as you might want to try using commonly available everyday neoprene rubber o-rings, you should be using a more oil proof and more heat tolerant Buna-N or Nitrile type of o-ring. They are often green or brown. If you get aftermarket ones that are black you may not know what they're made of, so probably best to avoid those.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now