The Strogg Oppressor can’t hand out Ammo like the GDF Field Ops, this is true. But while the Oppressor can’t provide his teammates with Ammo, he can plant a Tactical Shield (an orange hemispherical energy field) to provide them with temporary cover, and there’s no GDF equivalent for that.
Other than that, the Oppressor’s Bombardment deployables (Rail Howitzer, Plasma Mortar, Dark Matter Cannon) do the same damage as the Field Ops’ Fire Support deployables (Artillery Gun, Rocket Artillery, Hammer Launcher). Both classes can also call in remote firepower with a chuckable item: the Field Ops calls in Airstrikes with a smoke marker grenade, the Oppressor calls in Orbital strikes with a Stroggy beacon sort of thing.
As you can imagine, the Oppressor and Field Ops play differently, but they’re both kept equally busy.