Lope Y Fernandez was last seen filling the runner-up spot behind Palace Pier in the Queen Anne at Royal Ascot, while Breeders' Cup Mile hero Order Of Australia opened his account for the campaign in the Minstrel Stakes at the Curragh last weekend.
O'Brien said: "Order Of Australia has come out of the race (on Sunday) well. He cantered today and yesterday and he's very well.
"It's very possible that he could go. Obviously when he only ran at the weekend, we'd like to get him back in full work for four or five days and do the right thing for the horse.
"We're looking forward to Lope Y Fernandez. We have the choice of running here or going to the Lennox Stakes, but at the moment we're thinking of the Sussex. He worked this morning and we were very happy with him."
Battleground, who won the Vintage Stakes at the Qatar Goodwood Festival last summer, is a much bigger price than his two stablemates, having been well beaten on his latest outing in the Prix Jean Prat Deauville.
However, O'Brien feels he is better judged on his third place in the St James's Palace Stakes at the Royal meeting last month.
He added: "He ran a very big race in Ascot. He was a little bit disappointing the last day, but he was drawn out on a wing and he's a horse that likes to get cover.
"We've been very happy with him since and we think Goodwood will suit him - we think he'll leave that run behind him and come right back to his Ascot run and better.
"It's very possible that he could go to the Sussex as well."
It is 21 years since the popular Giant's Causeway provided the Ballydoyle handler with his first win in the prestigious Group One contest, a success he still recalls fondly two decades on.
The record-breaking Rock Of Gibraltar (2002), Henrythenavigator (2008), Rip Van Winkle (2009) and The Gurkha (2016) have all since added their names to the roll of honour in a race O'Brien ranks highly in the Flat racing calendar.
"Giant's Causeway is a horse that would come to mind when you think of the Sussex. It's a very difficult race to win," said the trainer.
"It's a very prestigious mile race. It's the ultimate test really - it's up and it's down and it's left and right. They have to have speed and stamina and they have to be very versatile.
"It can be the ultimate test of a miler, physically and mentally. It's a very important race in the pedigree of a horse going to stud."