It was a close fight, but as you mentioned, Petr's face was covered in blood and O'Malley seemed to be the one landing the good strikes. Petr really only hit O'Malley good once, which was enough to leave him looking like he was concussed in the post fight interview. With a fight that close though, when one fighter's face is covered in blood and the other fighter seems to be landing the big shots and has the momentum, I'm not surprised the way the decision went. I think if there was another round or 2, O'Malley might have been able to knock him out. I'm not so sure I agree with everyone saying O'Malley won the first round though. I think the first and second rounds went to Petr, but it's hard to discount all the major blows and blood landing on Petr...
Personally I think fights like this they should just give them a draw. I really couldn't pick a winner out of that fight at all. If you put a gun to my head and made me score it on points Yann should probably have won, but like people are saying Sean did more damage, but even he looked shocked when he announced it. Most people seem to be saying Yann was robbed though. I heard Joe Rogan say they should probably have more than three judges; somewhere between 5 and 10. Ten seems a lot but when it's just down to three they seem to get a lot of calls wrong and I'm sure favouritism plays a part in a lot of fights as well.
There is a site mmadecisions.com that gathers independent judges decisions on UFC events. A lot of judges decided, that Petr Yan won that fight. This fight might a nominee for a robbery of a year. Dont know if who did more damage is a correct criteria to decide who is a better fighter. A fighter can get a cut in first round and flood whole octagon with blood, but be a dominant in every round, or concentrate on body shots. Should he be considered a looser then?
Draw could be a best decision in this fight. Draw is a very rare outcome in UFC. There were like less than 50 draws in whole UFC history. Draw would be possible, if Petr was not top1 and Sean not top11, or if they both were not so big star in UFC.