OKW's stronger vehicle line-up and Ober synergy compensate for weak volks far better than Pgrens and OST's comparatively bad vehicles.
I'd argue that OST has a stronger lategame vehicle lineup than OKW does, but I can see this being up to personal opinion/playstyle.
The Brummbar and Pwerfer give OSTheer a distinct edge against infantry that OKW sort of lack in their vehicle offerings, which is kind of what the "weak" volks would be wanting to deal with. In terms of fighting vehicles, OST's infantry AT is superior to OKWs (nondoctrinally), Shreck Pgrens clearly outpacing a Shreck'd Sturm. Stug vs JP4 is a debate that could go either way, they both have their strengths and weaknesses.
OKW's big vehicular gimmick is the KT, which isnt exactly an unit used in every game.
Ober synergy is great, I agree, which is why I'd suggest Volks lean more heavily into supporting future Obers rather than being a "direct combat" unit that the STG upgrade seems to want to make them into. Obers' greatest flaw at the moment is their timing. By the time you can build them you are likely to have a "full" infantry roster, and at least personally I have trouble fitting Obers in without the expectation that I'll lose a couple Volk squads... which honestly I don't always have happen. I can't see Obers coming /sooner/ than they do now, though, as they're so incredibly powerful.