Galileo has been installed favourite for the Vodafone Derby after landing the Ballysax Stakes in commanding style.
The Sadler's Wells colt is 8/1 (from 10's) with Ladbrokes & Paddy Power for the Epsom classic after leading early in the straight to beat stablemate Milan by 3 1/2L.
'He must be very good as they are all as big as bulls and getting very tired on that ground,' said O'Brien, who had earlier saddled three beaten odds-on favs.
The Derrinstown Derby Trial back here next month is now on the agenda.
O'Brien and Mick Kinane doubled up half an hour later when Pebble Island (1/3 to 4/5f) made a successful debut in the 10f maiden.
The American bred colt was sent on inside the final 1/4m, and driven out to beat Alassio by 3 1/2L.
'He's a grand, big horse, and could be a Derby horse,' said O'Brien.
Rebelline is a live contender for the Irish 1,000 Guineas after taking the 1,000 Trial Stakes in runaway fashion under Declan McDonogh.
Kevin Prendergast's charge was always travelling well in this 7f Listed event, and stretched clear inside the final furlong to beat Imagine by 5 1/2L.
'She has been working really well, and that run will bring her on a bit. She will now go straight for the Irish 1,000 Guineas', said Prendergast of the full-sister to Quws.
He added: 'She likes a cut in the ground, and could be put away after the Guineas for an autumn campaign'.
Prendergast was completing a quick double having taken the opening 2yo race of the season with Master Papa.
Stephen Craine's mount was switched off the stands' rail just in time to pip Marionnaud and Agnetha in a blanket finish, and survived a stewards enquiry.
'He cost 32,000 at the Goffs Orby Sale, and needs at least six furlongs', said the Curragh trainer.
Dr Brendler gave bookmakers further reason to welcome the resumption of racing when causing a shock in the 2,000 Guineas Trial.
Mozart (1/4f) bidded to make all but found little from the furlong pole when headed by the John Hayden-trained winner and Derivative.
Dr Brendler was only pushed out by Fran Berry to hold on by a neck, with Mozart 3L adrift in third.
'He's only a bag of bones!' quipped Hayden afterwards, before adding: 'He's a big, rangy colt, and should make improvement through the season'.
'He will go next for the Tetrarch Stakes at the Curragh, and then hopefully the Guineas and Derby. Fran thinks he should stay at least ten furlongs', said Hayden.
Berry was completing a double having earlier landed the 10f handicap on Doonaree, who was patiently ridden to beat Moving On Up by 1 1/2L.
The Irene Oakes-trained Baggios Best (Tadhg O'Shea) led early in the straight of the 7f handicap, and held on gamely to beat Marefonic (5/1-4/1f) by 1L.
The 20/1 chance ensured the jackpot pool of 10,000 was carried forward to the meeting here tomorrow.
