The daughter of Galileo was as short as 3-1 for the first fillies' Classic of the campaign after winning three of her five starts as a juvenile, completing her hat-trick in the Cheveley Park Stakes at Newmarket in September.
However, O'Brien revealed his charge suffered a minor setback recently and she appears to be losing the race to line up on the Rowley Mile on May 6.
O'Brien told At The Races: "Because of the little hold-up she had, we had to go easy for a week or 10 days and we would have to force her to get her to Newmarket.
"She's in full work at the moment, but it's probably going to come a bit too quick for her."
O'Brien confirmed both horses are set to run at Newmarket early next month.
He said: "That is what we're looking at at the moment.
"We have another couple of weeks to go, but he (Saxon Warrior) is progressing along lovely.
"We never thought he was short of pace."
Even by his remarkable standards, the first weekend in May could be huge for O'Brien, with Mendelssohn firmly on target to run in the Kentucky Derby on May 5.
There has been plenty of discussion about whether Ryan Moore will ride at Newmarket or in America, but O'Brien is in no rush to finalise plans.
"I try to take one day at a time. Hopefully we'll have runners in the two Guineas and hopefully Mendelssohn will go to America," the trainer added.
"We'll usually decide those things a week before, or when their serious work is over."
Mendelssohn earned his trip to Churchill Downs with a jaw-dropping performance in the UAE Derby at Meydan last month and O'Brien is delighted with how he has recovered from his Dubai exertions.
He said: "He's in good form. Obviously he's had an easy couple of weeks and he came back well.
"Everyone seems happy with him at the moment. In the next couple of weeks his work will be stepped up, but it's so far, so good."