The former title-holder secured three winners at Leopardstown but O’Brien has taken the overall honours 87-84.
The Irish Flat season does not end until next weekend but both riders will be in America for the Breeders’ Cup.
O’Brien was on duty for his father, Aidan, at Doncaster where he guided Kingsbarns to victory in the Racing Post Trophy.
“Obviously it’s great and I’ve been very lucky to be able to ride some very nice two-year-olds and maidens along the way,” he said.
“I get to ride some very nice horses and I’d like to thank Mr and Mrs Smith, Mr and Mrs Tabor and Mr and Mrs Magnier and my dad for the opportunity.”
Smullen, champion jockey six times, was in fine form at Leopardstown where Famous Name was the highlight of his treble for trainer Dermot Weld.
Famous Name’s two-year-old full sister Big Break was another of Smullen’s winners. The 10-11 favourite gave a pleasing performance in the Group Three JRA Killavullan Stakes.
The jockey’s other victory came on Rasmeyaa (11-8 favourite) in the Irish Stallion Farms EBF Fillies Maiden.
Smullen’s agent Kevin O’Ryan reflected on a fine season. He said: “Pat’s had an unbelievable season. He’s won a lot of big races and hopefully he’s not done with yet.
“Pat is riding Speaking Of Which at the Breeders’ Cup on Friday, then he flies on to ride Galileo’s Choice in the Melbourne Cup.
“At Ascot last week he gave his two winners (Rite Of Passage and Sapphire) fantastic rides and it was a brilliant training performance by Dermot Weld.
“Pat Smullen is the ultimate professional and a great ambassador for the sport.”