The Galileo colt had previously finished third behind Harzand at Epsom and while he got closer at the Curragh, he was held off by half a length.
O'Brien has given him a break since that outing and expects to him to progress for the outing.
"He's in good form. He's had a good break since the Irish Derby and is just ready to start again. He should improve plenty for the run," said O'Brien.
The Ballydoyle maestro also runs Housesofparliament, who is the mount of Jamie Spencer and won the Bahrain Trophy at Newmarket last time out.
He had previously finished fourth behind the reopposing Across The Stars at Ascot.
"He's a very solid horse and is in good shape," said O'Brien.
"He had a good run in Newmarket. He stays well and handles good ground."
The Major General gives O'Brien a third string to his bow having been narrowly beaten by Ulysses in the Gordon Stakes at Goodwood.
"He's in good form. He had a very good run in Goodwood and we think he is a horse that is still progressing," O'Brien concluded.
Sir Michael Stoute's Across The Stars cut no ice in the Derby but bounced back to win the King Edward VII Stakes at Royal Ascot.
The Sea The Stars colt has to concede 3lb to all of his rivals after that Group Two success.
"He is in good form. I hate running him with that penalty, but it's a race for him at this stage because I wanted to give him a good break after the King Edward," said Stoute.
He is owned by Saeed Suhail and his racing manager Bruce Raymond said: " It was a bit of a relief to see him win at Ascot after everything went wrong in the Derby.
"It would be wrong to say that Michael was confident, but he thought he would go very close. He was certainly more confident than I was, though.
"He's working better than ever and has done well physically.
"You'd think he should be first or second on form.
"He's in the St Leger but we'll see what happens here first."
The two completing the field are Mick Channon's Harrison, a close third to Housesofparliament in the Bahrain Trophy but a big disappointment in the Gordon Stakes and Roger Charlton's Imperial Aviator, hugely impressive in the London Gold Cup but a flop in the French Derby.