It is going to hurt, but the only way out of the pain, is through it. Days will turn into weeks, which will turn into months and one day, you are going to look up and say "I was fighting for her. Fuck that b!tch." If you are really lucky, indifference will settle in quicker as anger will mean you still have feelings for her. The goal is indifference, but you got to go through this painful time to get there.
I also contacted the GF of OM and she suspected. When she confronted, he denied it, said they were just close friends, she chose to believe him but said she doesn't trust him and will let me know if there is any contact between WW and OM.