Pressing the o key causes your character to autoexplore the current floor, mapping out unexplored territory until something of interest is discovered, such as monsters, items, traps, or unusual dungeon features. This feature is primarily included for convenience's sake, allowing you to rapidly breeze through uneventful areas, but it does offer some unique advantages and disadvantages:
Pros: Autoexplore's biggest benefit lies in the fact that it forces you to pause after discovering an enemy. Careless manual movement may result in you taking a step or two before reacting to the new threat, wasting your opportunities to attack at range or flee, and possibly giving the enemy free attacks against you. This is especially true when opening doors or turning blind corners. This advantage is meaningless if you're paying careful attention, but fingers do slip on occasion.
Cons: Autoexplore does not explore in a terribly efficient manner. Oftentimes, it leaves the corners of rooms or ends of hallways unseen until after exploring every other part of a floor, forcing you to trudge around the floor once more to explore every tile. Of course if you don't intend to explore every tile in a certain area, or if you don't care about your turn count, autoexplore works fine.
Also, autoexplore hits some technical hurdles in certain branches. In particular, the tides in the Shoals result in erratic behavior, and autoexploring the Abyss results in a lot of fruitless backtracking as you re-explore areas you just forgot about.
Autoexplore can't be used in the Labyrinth.