I have no idea how to get OKW to function in 1v1 anymore, but from what I see in tournies and top players, you need to be aggressive with your sturms early, especially vs USF since they start with a sizeable disadvantage. A light vehicle is required a lot of the time to help your volks through mid game. raks are always near your light, and don't try and fight upgraded allied infantry 1v1 with volks unless its con ppshs at max range.
Sturms are the saving grace of okw early game vs usf. They beat all their units in 1v1 scenarios and are good for grabbing key garrisons (which usf are very weak against) and keeping riflemen from moving up on your volks.
The kubel is also very nice as usf has no automatic weapons at the beginning of the game or you could just cap the other side of the map with it and focus your entire army on his side.
If he goes lieutenant he has not at guns, so FHT or luchs, and no good indirect, so leigs are also good. If he goes captain he has absolutely no suppression so playing infantry heavy is to your benefit there. It’s also very doable to hold off on choosing commanders until he picks a tier and then picking based off what he will have or not (ex. going luftwaffe because he went captain and has no suppression to deal with falls).
OKW are strong when you blob and rush light vehicles, continually camping the cutoff of your opponent. Cmd panther or jadgtiger are strong. Rushing puma with (Heat) shells allow to 3 shot medium vehicles.
Doesn’t puma HEAT take 4 shots to kill mediums? Unless you mean something other than tanks.