Here's why:

* Valency refers to the combining capacity of an element. It's essentially the number of electrons an atom needs to gain or lose to achieve a stable electron configuration.

* Oxygen has 6 electrons in its outermost shell. To achieve a stable octet (8 electrons), it needs to gain 2 electrons.

* This "need to gain 2 electrons" is what makes oxygen's valency 2.

Let me know if you'd like to explore other aspects of oxygen or chemistry!