I have seen numerous mentions of "the Overgod" Ao but other then being obscenely powerful, I have not found any information on him.
So, who is Ao and what is his portfolio?
This is found in the Faiths and Avatars sourcebook (p. 30):
(Lord Ao, the Hidden One)
Over-power of the Cosmos and Realmspace
PORTFOLIO: Creation of deities, maintenance of cosmic balance
Before the Time of Troubles, the question of who the gods answered to was a philosophical conundrum better suited to sages than priests. During the coming of the Avatars, however, it was revealed that the powers themselves had a deity, or at least a more powerful power who had the ability to chasten and punish them for their actions. This entity is Ao (AY-oh). He was present during the Time of Troubles, yet since that time has retreated to his home plane, dimension, or existence, and has not responded to mortal entreaties since (though one paladin claims to have seen him in a vision). [...]
Ao is the only over-power in Realmspace. Ao lies beyond the bounds of the rankings of deities’ powers in any way. He is considered to be more powerful than any other power and all powers combined. Ao has little concern for day-to-day mortal life, but he regards the powers of the Realms as his creations. He requires no worshipers and grants no spells to mortals. It is doubtful that he even listens to mortal prayers or pleas. He serves as a watcher and a guardian over the Realms from outside of its cosmos.
There is further information about the avatar (only seen once), but basically he is outside of everything.
A good amount of information can be found in the Forgotten Realms wiki entry for Ao. To quote just the beginning of the wiki entry (it is far too large to reasonably quote everything in this answer):
Lord Ao, known as the Hidden One or the One Who Is Hidden, was the Overgod of the worlds of Abeir-Toril. As Overgod, all deities and primordials of Abeir and Toril, even those who also operated in other spheres and planes, such as Lolth, were subject to him.
The wiki entry includes details on history, personality, etc of Ao. As for your specific question of Ao's portfolio, it is only listed for the 2e tab as: "Creation of deities, maintenance of cosmic balance".
Being a wiki, this is fan-made and effectively anyone can contribute which may make the information not perfectly accurate. Fortunately, the article also includes a number of references from which the information was taken and that may provide more officially published details on Ao.
When I was DM in DND 3.5 I also came across this deity. After reading his description I saw parallels to the slavic great deity "Rod" which is essentially the whole universe. Because he is everything, when someone worships him they are not only worshipping the good but also the bad. Consequently, aspects of "Rod" became their own deities which can be worshipped without also worshipping the "other side".
As I understood it the same concept applies to Ao and the other D&D Deities are "just" aspects of Ao (Alpha and Omega...)