O’Sullivan booked his place in the final with a 6-4 battle.
O’Sullivan was cruising to begin with as he lead 4-0 after breaks of 61, 54, and 64.
Maguire battled back to 5-4 before O’Sullivan saw him off.
He will meet Shaun Murphy or Ryan Day in Sunday’s best-of-19 final.
“He came back at me but I was lucky to be 4-0 ahead,” O’Sullivan told BBC Sport.
“He didn’t capitalise on the first few frames and I did well to stay in and am pleased to reach another UK final.
“I knew he was playing well because I saw his results and he looked to be cueing well. I knew I had to play decent to get through.
“He looked to be cueing pretty good so I knew I had to play decent today and luckily I was just able to do enough really.”
O’Sullivan also survived a scare in the fourth round as he beat Thai youngster Sunny Akani 6-5.
He will now face the winner of Shaun Murphy vs Ryan Day in Sunday’s best-of-19 final.