I was searching some informations about OLL probabilities and found that on SpeedSolving forum from a British guy (Mark Rivers):

Here is a list of all the OLL shapes, ordered by the average number of moves they save compared to solving the same cases with 2-look OLL. Learning full OLL in this order is a good idea because the ones nearer the top will cut more time off your solves, on average.

10.00   Stealth
 8.50    Squares
 8.13    Dots (1/54)
 7.50    T shapes
 7.31    Small lightning bolts
 6.67    P shapes
 6.00    Kites
 5.88    C shapes
 5.58    L shapes
 5.56    Knight moves
 5.38    Fishes (excluding kites)
 5.25    Big lightning bolts
 4.25    H
 4.13    I shapes
 3.81    Dots (rare)
 3.44    Awkward
 2.63    W shapes

In fact, learning just the top 15 OLL algs delivers 42% of the total move savings of learning full OLL, so if you only want to learn some of it, those are the ones to go for.

If I learn OLL one day, I think I’ll memorize cases in this order, it’s smart.
The original post is here.