Century City landed his second Guineas trail this season but only just when holding off stablemate Creekview by a shorthead in the featured Tetrarch Stakes.
The Danzig colt looked set to land this Group 3 event in convincing style when striking the front over a furlong out, but needed to be driven right out by Mick Kinane to prevail.
O'Brien felt afterwards that Century City had idled in front, and indicated that both colts could have a rematch in the Irish 2,000 Guineas.
The O'Brien/Kinane team suffered a disappointment in the Mooresbridge Stakes where Milan was pulled up lame approaching the home turn.
The son of Sadler's Wells was found to be lame behind, and was taken to the Troytown Veterinary Hospital for further examination.
Rebelline took advantage in workmanlike fashion for Declan McDonogh, only getting on top of the front-running Shoal Creek in the closing stages.
Kevin Prendergast nominated the Group 1 Prix d'Ispahan over 9f at Lonchamp on May 19 as her next target.
Solid Approach initiated a double for McDonogh and Prendergast in the 10f handicap, sweeping to the front well over a furlong out to beat Margarula by 2L.
Rahn (4/1-5/2f) was all the rage here but could never get on terms and was soundly beaten in seventh.
Ontario headed Pakhoes inside the final furlong to score by a head in the 2yo maiden, with the pair pulling 7L clear of the remainder.
Louisville completed a treble for O'Brien and Kinane when staying on well inside the final furlong to make a winning debut in the 1m maiden.
Dermot Weld and Pat Smullen got their names on the scoresheet when Rum Charger took the 1,000 Guineas trial, the Athasi Stakes, from Kournakova.
The 7f handicap looked a devilishly difficult puzzle beforehand, and it took a photo-finish to seperate Anne-Lise, Top Star and Cool Cousin at the line.
