The OLL is the second last step in most methods. This is where you prepare the last layer pieces to be placed into their final positions by orienting the pieces (making them all face up). Basically, you're going to make the top face look like one solid color, but the top layer isn't necessarily going to be finished at the end of this step.
A beginner only needs Bar 02, L 02, and Sune 01 to finish their OLL (cross then corners), but it wouldn't hurt to know some of the other sunes! It would certainly pay off to learn a couple of them since they are so common. Beginners... don't even think about trying to learn all of these. Use that ambition to learn more PLLs.
Intermediates should know how to make a cross, and all of the sunes. This will give you a 2look OLL (see section below). When you think that you are ready to start learning a lot of OLLs, take this advice: the hardest part about learning OLLs is getting started. It seems like there are way too many, but learn a couple at a time and before you know it you'll know a large portion of these.
Advanced solvers should know many or all of these. If you do know your full OLL, please compare some of these algorithms to some of your least favorite cases. Maybe you'll find one you like more than your own!
Sune 01 R U R' U R U2 R'
Sune 02 R' U' R U' R' U2 R
Sune 03 R U2 R' U' R U R' U' R U' R'
Sune 04 R U2 R2 U' R2 U' R2 U2 R
Sune 05 R' U' R U R' F' R U R' U' R' F R2
Sune 06 R U R' U' (R U' R' U2)*2 R U R'
Sune 07 r U R' U' r' F R F'
Bar 01 R U R' U' M' U R U' r'
Bar 02 F R U R' U' F'
Bar 03 R U R' U' R' F R F'
Bar 04 R' F R U R' U' F' U R
Bar 05 L F' L' U' L U F U' L'
Bar 06 R U R2' U' R' F R U R U' F'
Bar 07 R' U' R' F R F' U R
Bar 08 r U' r' U' r U r' F' U F
Bar 09 R' F R U R' F' R F U' F'
Bar 10 l U M' U L' U' l U' l'
Bar 11 r' U' M' U' R U r' U r
Bar 12 R U2' R2' U' R U' R' U2 F R F'
Bar 13 F R U R' U' R F' r U R' U' r'
Bar 14 f (R U R' U')(R U R' U') f'
Bar 15 R' U' R U' R' d R' U R B
Dot 01 r' R U (R U R' U') r2 R2 U R U' r'
Dot 02 r' R2 U R' U r U2' r' U M'
Dot 03 l L2' U' L U' l' U2 l U' M'
Dot 04 R U R' U R' F R F' U2 R' F R F'
Dot 05 r' R U R U R' U' r R2' F R F'
Dot 06 R U2' R2' F R F' U2 M' U R U' r'
Dot 07 F (R U R' U') F' f (R U R' U') f'
Dot 08 R U2' R2' F R F' U2 R' F R F'

L 01 r U R' U' R r' U R U' R'
L 02 f R U R' U' f' y2 F U R U' R' F'
L 03 f' L' U' L U f
L 04 R d L' d' R' U R B R'
L 05 L' d' R d L U' L' B' L
L 06 R U R' U R U' R' U' R' F R F'
L 07 L' U' L U' L' U L U L F' L' F y2 R' U' R U' R' U R U l U' R' U x
L 08 r U R' U R U' R' U R U2 r'
L 09 l' U' L U' L' U L U' L' U2' l
L 10 F (R U R' U') (R U R' U') F'
L 11 r' F' (L' U L U') (L' U L U') F r
L 12 r U' r2 U r2 U r2 U' r
L 13 r' U r2 U' r2 U' r2 U r'
L 14 r U2' R' U' R U' r'
L 15 r' U2 R U R' U r
L 16 r U R' U R U2' r'
L 17 r' U' R U' R' U2 r
L 18 r' R2 U R' U R U2' R' U M'
L 19 l L2 U' L U' L' U2 L U' M'
L 20 F R U' R' U' R U R' F'
L 21 R U2' R2' F R F' R U2' R'
L 22 R U R' U R' F R F' R U2' R'
L 23 R' U' l z' R U' R' F R U R'
L 24 y' (R' F R F') (R' F R F') (R U R' U') (R U R')
L 25 y (R B' R' B) (R B' R' B) (R' U' R U) (R' U' R)
L 26 r U R' U' R r2' F R F' M'
L 27 l U R' D R z'x r2' F r U2' l'

A 2look OLL is a great way for intermediate solvers to boost their average speed without having to learn a ton of new algorithms. By using a 2look OLL you can reduce the number of algorithms that you need from a seemingly impossible 57 down to a cool, easy to manage nine. Here's how it works: 2looks means there are two seperate recognitions. Firstly, you look at the edges and recognize them as one of 4 cases; a bar shape, an L shape, a dot shape, or a cross shape.You know from the beginners method how to make any of these shapes into a cross, which is another way of saying you solve the edges. Secondly, you look at the corners and recognize one of 7 possible sune cases, and perform the appropriate algorithm. Essentially a 2look OLL means you know all of the cases for corner orientation, which works out to 7 algorithms (The "Sune" cases above).
Please keep in mind, however, that although learning this method will shave several seconds off your average; full OLL is much faster. You should still try to learn full OLL at some point, even if you do know a 2look OLL system. An algorithm every other day and you'll know them all in a few months. Also, the OLLs are much easier to learn than the PLLs, because they are generally shorter and have common triggers.
Want to see more? Here's my channel: www.youtube.com/user/lancetheblueknight