The win was one of the biggest shocks in the tournament’s history.
The unranked debutant Cahill, who is the first amateur to play at the Crucible, beat O’Sullivan 10-8 to advance to the second round at the expense of the world number one.
The 23-year-old Cahill lead 5-4 overnight but O’Sullivan, who has 36 ranking titles and is nicknamed ‘The Rocket’, clawed his way back to stay in the hunt.
He then missed a final pink in the 17th frame that allowed Cahill to take a 9-8 lead.
O’Sullivan then fouled while attempting to split the pack in the next frame.
It gave Cahill the chance to clear the table with a break of 53 to win the match and cause a major upset.
“I could barely stand up at the end,” Cahill said after his historic win. “I scored a good pressure 70 to go 6-5 up and after that I felt like he was the one under pressure. He didn’t want to lose to me.
“I have always believed in myself and that I can beat anyone on my day. I want to show what I can do now.”
O’Sullivan, who seemed to be rushing through his game which led to errors, cited illness as the reason for his first round exit.
“I haven’t felt great for a few weeks. All my limbs feel really heavy,” O’Sullivan told reporters. “I feel absolutely shattered and drained, I was struggling to stay awake.
“You have to come here feeling physically and mentally good. You don’t expect to do well here but I came here trying to do my best.
“I tried to hang in there and tried to do as much as I could to see if I could get through this match and have a few days off and feel better. But it didn’t happen.”
However, the 43-year-old, who became the first player to compile 1,000 career century breaks last month, gave Cahill due credit for the victory that came under immense pressure.
“He’s come here and played brilliantly,” O’Sullivan added. “He did well, he held himself together.”