O'Brien is just one winner away from equalling Bobby Frankel's record of 25 victories at the highest level in a year and Johannes Vermeer was well supported for this 12-furlong heat.
Johannes Vermeer was beaten just a head in the Ladbrokes Stakes last Saturday and Ben Melham was happy to settle in the chasing pack as first Amelie Star and then Sir Isaac Newton took the field along.
Sir Isaac Newton, who was trained by O'Brien until this year, built up a healthy lead of 10 lengths with around five furlongs to run, but he was quickly reeled in before the top of the straight with Boom Time poised to challenge while Johannes Vermeer was on his heels.
When Cory Parish gave the signal, Boom Time kicked on with about a furlong to run but Johannes Vermeer could not quite follow him through, eventually finding a gap to narrowly lose out on second to Single Gaze.
Hughie Morrison's Melbourne Cup hope Marmelo ran an eyecatching race in the hands of Hugh Bowman, making up plenty of ground in the straight to finish just out of the frame.
Parish, who was riding his first Group One winner, said: "I got to about the 50 (metre mark) and I knew I had it in the bag, he was travelling so well, and I just thought all my dreams were coming true.
"I just can't believe it."
Boom Time is jointly trained by David and Ben Hayes along with Tom Dabernig and Ben Hayes was thrilled to have landed his first Caulfield Cup on a horse also owned by his family.
He told racing.com: "The family own the horse - it was amazing. I just remember him getting out (of the stalls) and then it's all just a bit of a blur.
"It was a great run, fantastic for Cory. He works really hard for us, it's just so exciting for him and the family.
"I'm the third generation of the Hayes family to win a Caulfield Cup now, so it's just super exciting. It's just great."