Gold Cup hero Leading Light made the perfect return to action in the Palmerstown House Estate Irish St Leger Trial Stakes.
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Given a break since his efforts at Royal Ascot, last year's Ladbrokes St Leger winner was shouldering 10st against a useful field, but in the end just needed to be kept up to his work to win.
The front-running Shu Lewis narrowly missed out for second as Royal Diamond finished well, but Leading Light (2/5 favourite) was never in danger once he got his in front around a furlong out.
The Montjeu colt took his carer record to eight wins from 10 starts, being beaten only on his two-year-old debut and in the Arc.
O'Brien said: "I'm delighted with that. He's had a good break since Ascot. We let him down well after that and it's nice to get him started back.
"He came here two weeks ago for a canter to wake him up a bit. He should come on nicely from that as he's giving a good blow.
"The (Irish) Leger falls lovely in the second half of the season for him. I don't know where he'll go after that but the main thing is the lads would like him back for Ascot next year.
"He ran in the Arc last year, but was back a long way. They'll decide where he goes."
Leading Light's trainer Johnny Murtagh said he was pleased with the performance of his charge, who will now return to Ascot to bid to take the same contest he captured in British Champions Day last year.
Bookmakers Sky Bet cut third-placed Streetcar to Stars from 20/1 to 14/1 for the St Leger at Doncaster after his solid effort.