Selby admitted it would mean "the world" to him to join that group.
"I knew it was going to be a tough task but, in only my second time here to get back into the final, even if I lose I've achieved more than I expected," he said.
"It's always harder to defend a title, I felt that coming here this year. I put a lot more pressure on myself than I normally do going into a tournament."
Maguire, who had been so impressive in beating Neil Robertson in the quarter-finals, managed a highest break of only 44 against O'Sullivan and looked completely out of touch.
The only frame he won was the third, when O'Sullivan gave away 16 points attempting to get out of a snooker, eventually leaving Maguire a free ball.
The Scot was in first in the fourth frame but managed just 28 as O'Sullivan stepped in with a break of 84 to lead 3-1 at the interval.
The world champion then stepped up a gear to take the next three frames as Maguire struggled to get back into the match.
"I only had an hour's practice, I'm still not fully comfortable with the cue but I'm managing to have a heart, be determined and play with the attitude to win and just try to enjoy my game," said O'Sullivan reflecting on his win.
"I would like to be a bit more clinical but I can't be too hard on myself, that was pretty decent all round."
Five of Selby's previous six matches in the Masters had gone to the final frame but once his nerves settled, he wasted little time in getting down to work.
A century put him 3-2 up and after taking the sixth, a break of 82 put him within one frame of victory.
He fell short of another century in the eighth but had done enough to win.
"I was so nervous before the start of the match, it was a massive game and I just couldn't get settled," Selby said.
"To be 2-2 at the interval was a blessing because I could easily have been 4-0 down. After the interval I seemed to get into the zone and be more relaxed."
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