I've always wondered. It was 1989. The writers at the time did not know what to do with the Borg. The end scene of "Q Who" airs. Guinan warns Picard that because the Borg know of the Federation's existence, that they will be coming. The Borg in that episode did very destructive things. If it wasn't for change to assimilation, would it have been oppression and conquest?
It would not have been conquest. In the first appearance of the Borg, Q is very clear that this is not their goal.
Q: The Borg is the ultimate user. They're unlike any threat your Federation has ever faced. They're not interested in political conquest, wealth or power as you know it. They're simply interested in your ship, its technology. They've identified it as something they can consume.
In the original conception of the Borg, they were mainly interested in technological assimilation, not biological. Their plans for Earth would have been the complete physical removal of all its technology, and the incidental destruction of human civilization on the planet due to such serious damage.
WORF: Captain, the sixth planet in the system is Class M.
DATA: There is a system of roads on this planet, which indicates a highly industrialised civilisation. But where there should be cities there are only great rips in the surface.
WORF: It is as though some great force just scooped all the machine elements off the face of the planet.
This is also what they did to Guinan's planet:
GUINAN: My people encountered them a century ago. They destroyed our cities. They scattered my people throughout the galaxy.
In later conceptions, the goal of the Borg quickly changed to assimilating humans as well as technology. This is closer to conquest, obviously, but is nonetheless distinct. The Borg do not desire a subservient civilization to provide labor or to oppress for their own amusement. They instead seek to assimilate people physically into the Collective and become entirely different people, in the process entirely destroying their civilization, albeit retaining any useful elements, and effectively destroying the minds of that civilization's members. It might more properly be considered closer to genocide, although the individuality of some members can be recovered if they are removed from the Collective.