O'Connor, who is the only debutant in the field at The Crucible, amassed a 7-2 lead during the first session on Sunday and held on for the biggest win of his career despite a rally from one of his snooker heroes.
Defeat capped a dismal season for Selby, who has slipped to fifth in the world rankings.
"I will take a long time to think about it over the summer," said the 40-year-old.
"Away from snooker I'm happy, then when I come to snooker it's the opposite. It was pathetic from start to finish."
"I'll sit down with (my wife) Vicky and see what the options are," added Selby.
"Obviously it will be a big decision but if I do carry on playing I need help, probably on the mental side of it a little bit more just to go out there and enjoy the game.
"That's all I want to do, go out there and enjoy it and for me, it's always sort of life or death; I've always been that kind of character, putting too much pressure on myself and trying too hard."
'I felt awful'
Selby was not the only one to label himself 'pathetic' - Gary Wilson said as much about his own game after losing 10-5 to qualifier Stuart Bingham.
"I just felt awful, flat and I really didn't want to be there," he told the BBC. "There are no positives. I didn't even feel good at the best tournament in the world.
"It was almost like I was having to think 'why is it like this' and I'm sick of battling. I wasn't up for the battle today, I tried a bit harder tonight but when you know you're not playing well enough you're just scraping around.
"Stuart played better than me, I was pathetic."
In another of Monday's matches, Shaun Murphy beat Haotian Lyu to set up a clash with Stephen Maguire in the next round. Lyu had started well to leave the pair deadlocked at three frames apiece before Murphy went on a run of five consecutive frames to eventually win 10-5.
