HMG34 is a cheap heavy machine gun unit (which has a high target size for infantry like other weapon teams, meaning bullets are more likely to hit them) that requires setting up and suppresses infantry, but is only barely able to kill them, generally enforcing it have amble quantities of support nearby to take care of that for them. Get them to control enemy infantry's movements and slow blobs - be sure to make use of its low cost by getting more of other units around it to cover it (or just more HMG34s) and kill what it suppresses.
Obers are an expensive mobile infantry unit that are specialized in killing infantry at long-range. With their LMG34 upgrade and low target size (meaning bullets are less likely to hit them), they are nearly outmatched in that regard, but their expensive cost means it is important to keep them away from vehicles which they are nigh helpless against. Get them to kill infantry, especially from a defensive heavy cover position surrounding open ground to enforce approaches against them will be very costly. Their smoke grenade can also be quite useful against infantry blobs, slowing and damaging them to an extent any one grenade could not hope to (it won't kill any of them by itself however), that thing's not too far from basically being suppression in an area that costs munitions.
The two units work all right together, with HMG34s controlling an area while Obers' DPS are not something would-be flankers want shooting at them while they run over open ground to an HMG34 - few infantry units are likely to be able to slip a grenade toss into an HMG34 crew while Obers are shooting at them. Both will require AT support, however.
Ostheer Pioneers have much weaker Mp40s than the Mp40 assault grenadiers use, etc.
I'm pretty sure Pioneers' MP40s are basically just Kar98Ks at close-range at best (which is still a potential improvement over Combat Engineers that are basically Scripts but only 4 out of 6 of them), while AGs' do have more DPS at best (which is obviously still not enough...).