ob

Ob takes the ACCUSATIVE and nothing else. Its oldest sense is physical, "in front of, facing", which survives in ob oculos, "before the eyes", and in the verb obicio, "to throw in someone's way". In ordinary prose it has gone causal: "on account of, because of".

It is rarer than propter and it clings to a handful of set phrases: ob eam causam, ob eam rem, ob hoc, and above all quam ob rem, "for which reason, why", which is worth treating as a single word. As with propter, it governs a NOUN. For "because" in front of a whole clause with a verb you need quia, quod or quoniam.

As a prefix it assimilates ruthlessly to whatever follows, and a great many English words hide it: ob plus capio gives occupo, ob plus fero gives offero, ob plus pono gives oppono, ob plus caedo gives occido. When you meet an English word in oc-, of- or op- and cannot see the root, this is usually why.