how does oxygen cause chemical weathering?

Answer 1

Oxygen by definition is an oxidizer. The atom (it is a base element in the periodic tables) will bind with almost any other element and in that process of binding, it creates an oxide of that element. It is the identical process of raw iron/steel rusting. What you refer to as weathering is actually the oxidation of the material.

Further, oxygen mixing with certain other chemicals and being reacted to by sunlight will create a compound called ozone. Ozone will attach rubber and other pourous compounds and eventually render them rotted9weathered)

Answer 2

It oxidizes things.

Rust is oxidation, for example.

Silver jewelry turning black is oxidation.