O'Malley's clear advantages in these fight are his range, height, and striking accuracy. He is a very coordinated striker who works best at tip range, who throws great combinations from different angles at great speed, can throw from his back foot, and who uses a lot of feints and mixups extremely well. He tends to fare better against less mobile targets who are at a clear speed and range disadvantage, like Moutinho, where he can style off and chain combinations or use his speed advantage to land counters. In his last fight, Pedro adopted a mobile strategy, to move in and out of range, avoiding the boxing exchanges, and poking with leg kicks that while mostly checked also added up. O'Malley had trouble finding his range, even if he did manage to land a few good punches, including one that shut Pedro's right eye. Upon rewatching I thought he won that round. He is exceptionally good at chaining strikes and finding the counter punches, and avoiding head strikes.
Yan is even faster, has better movement, and a much more polished striking game, not to mention speed, than Pedro. He works at his best moving in and out with fast short combinations. Nothing particularly fancy, just tenacious, fast, and precise. He has a big reach disadvantage, but he moves in and out very well, and tends to find his range and pick it up as the fight moves forward. He will probably move laterally a lot and frustrate O'Malley, who will have to try to find his range against a much more mobile target than ever before. While O'Malley generally has a speed advantage, against Yan this might not be the case. Propensity for injury might be a factor, but it's impossible to factor in such things. Neither fighter seems to opt for wrestling offensive, though both have good takedown and submission defense.
The thing with O'Malley is that he is in love with the idea of becoming a KO artist like Conor or Anderson, while he should grow into a more dynamic MMA fighter like DJ or GSP, who could change gears mid fight if something was not working. His BJJ is actually surprisingly good, but he rarely has alternative gameplans and always seems to prefer to strike. He needs to get his ego in check, and realize styling off with strikes won't always be available as he fights the higher level competition moving forward, if he wants to be consistent.
I think this is a difficult fight to call; far more than people are making it out to be, though obviously it is an enormous step up in competition for Sean. I'm going to go out on a limb and say O'Malley shocks the world.