"I've been fortunate enough to win plenty of big races in England and Ireland and did a couple of winters in Dubai, but it's my first time riding here in Australia," McNamara said.
McNamara is one of a string of jockeys O'Brien uses and is a former champion Irish apprentice.
He said he had ridden a couple of Group race winners for O'Brien and had been placed at Group One level but had not managed to break through at the highest level.
"Hopefully that will come some day," McNamara said.
"But I have had a good career so far, 150 odd winners and its been going OK.
"I get a few outside rides, but every morning I am in at Ballydoyle and riding work.
"I've been there for the last four years.
"I have had a great time there and am very grateful for Aidan and the owners giving me some nice rides."
McNamara has ridden some quality stock at Ballydoyle and partnered last year's Melbourne Cup runner-up Johannes Vermeer early in his career.
The jockey visited Caulfield this week for a look at the track and will have another walk around on race day.
"There's some good weather coming and I'd expect it would be good, fast ground on Saturday," McNamara said.
"It's quite a short straight. There's nothing similar, especially in Ireland.
"It's five-and-a-half furlongs around a bend, it should be quite straight forward."
McNamara is the regular trackwork partner of The Cliffsofmoher and was aboard when the Caulfield Stakes runner galloped at the Werribee quarantine facility on Thursday morning before jumping from the barriers.
"He gives me a similar feel to back in Ireland," McNamara said.
"We were happy with him when he left and he seems to have carried that form out with him."
Mark Zahra rides the horse in the 2000m-Group One race on Saturday.