I think it is Felicity's mom. When I first saw this-I thought Oliver was really cold and disconnected at the grave and he, FINALLY, said that something wasn't his fault (either meaning it really wasn't his fault or he has finally decided to stop taking the blame for every.single.awful.thing that happens in that city). Oliver didn't seem to be feeling guilty-but just resigned-like, I know what I have to do now-this man took everything from me (we obviously know he is going to do something against the kid and the mom-which will bring Felicity in on the secret and destroy their relationship, etc.) so Oliver is just resigned that this man has destroyed his life and he has to kill him. Felicity wasn't upset-she was pissed! Probably at many things-but I do think that it was still a reasonable reaction for her-other than just sitting in the car crying. Felicity gets mad before she gets upset-and I think she either had already been sad or still hadn't gotten there yet-but it being her mom is the only person I can really think of that would make her that angry at that point in time. I was really going for the kid (and I will probably go to hell for this-but if it is either the kid or the mom-I hope it is both-for 2 reasons-1 if it is just the kid-the mom is going to be all up in Oliver's face about it. 2. If it is just the mom-then Oliver is going to have to raise the kid-and I don't really want to see that dynamic on Arrow-sorry, but I don't). The ONLY reason I don't think it is the kid is because Felicity was more pissed than Oliver seemed to be pointing me to the fact that it is someone closer to Felicity than Oliver.