The Pirates dominated at Wimborne Road, with Richard Lawson scoring a 15-point maximum and Tobias Thomsen (9+3) remaining unbeaten until Heat 15.
All seven home riders secured at least one win or paid win.
Lawson said: “That’s what you go for every time but it’s tough, especially when you meet Sam Masters.
“I’ve said it before, I believe he is one of the best No. 1s in the league and to come up against him three times is tough.”
Poole Pirates speedway v Oxford Cheetahs  in BSN Series Semi final 1st leg at Wimborne Road. Heat 7 Anders Rowe (red) and Paul Starke (blue) (Image: Richard Crease) The Cheetahs had a challenging night, with Masters scoring 11 points from five rides.
Reserve Luke Killeen picked up 9+1.
Killeen said: “It was a tough night for sure, but I gave it my all.
“Overall, I feel I proved my worth to the team and I was happy with my performance.
“It wasn’t a great result for us tonight.
“But if they can do that to us here, there’s no reason why we can’t do that to them back at Oxford.”
The visitors were without Jonas Knudsen and Francis Gusts.
Gusts has been given a 28-day ban by the Speedway Control Bureau after competing in a meeting in Poland on Tuesday.
This was the day after he was ruled out of the Wimborne Road meeting due to reported illness.
The absence of these key riders may have contributed to Oxford’s defeat, but they remain hopeful for a comeback in the second leg at their home track.