OLL means Orient Last Layer. There are a total of 57 OLL algorithms to learn to orient the last layer in one step.
Up until this step only 7 OLL algorithms had to be learned since the orientation of the LL edges have been delt with in a previous step. Learning all 57 (-7) algorithms is really not that hard to do if you are motivated to learn them. It took me approx. 3 months to learn all of them (including writing the OLL pages on this site). I believe you already know the OLL algorithms for when all edges are already flipped correctly otherwise you have to learn them as well.
Learning these OLL algorithms save you the trouble of orienting the edges and thus saves you a couple of seconds in the total solving time (and makes cubing much more fun since you have to recon more cases).
I have given one algorithm for each possible rotation of the U layer. The "U0" means just apply the algorithm as stated. For the U, U' and U2 algorithms apply the corresponding twist on the U layer and then apply the algorithm for that case. My favourite OLL algorithm is always the U0 and is typed in a bold font.
There is no need to learn more than one algorithm for each case, I have given the other as "candidates" if you don't like my selected algorithm. Each cuber has it's own selection of algorithms they like best, maybe you like mine the best.
I believe no more introduction needs to be said, let's skip on to the different cases.
No edges flipped correctly & T-shapesFor a printable page of all OLL click here