Oblak is one of the best keepers I've seen.
Oblak by a clear margin, than Alisson, than Stegen very close to Alisson. Pretty much how Andresito rates it above.
Stegen is a world class keeper, but out of the 3, he receives the most goals where I say to myself 'a keeper of his quality should've saved that'.
Stegen is similar to De Gea in this way. Amazing saves at times and crazy reflexes, unbeatable when they are in the zone so to speak, but you know the mistake is coming, and it does not happen as rarely as it should be for keepers of their quality.
I think we can call that the consistency of making sure you get beaten only by superior finishing. It is very valuable for keepers. And both Stegen and De Gea let in goals where the opponent doesn't have to work too hard more often than keepers like Oblak.