Oa is the central hub for the Green Lantern Corps as well as the house for the Central Battery and Book of Oa. It is also said to be the center of the Universe.
From this wiki (which is the only place I've been able to find a location for Oa) it says that it's in the Milky Way Galaxy. Relatively close to Earth.
What is the distance from Earth to Oa?
Do the Green Lanterns protect the ENTIRE Universe, or just the obervable Universe (about 93 billion light years from the point of origin)?
How large are the sectors?
Were these questions ever asnswered in DC's Universe?