Manager John Farrell made the announcement following Monday's 12-5 win over the Rangers.
Buchholz (3-9, 5.91 ERA) was sent to the bullpen May 27 and restored to the rotation June 21, mainly due to the ineffectiveness of Boston's other depth options.
Also, Buchholz, who threw just 65 pitches Saturday, will be available out of the bullpen sooner than O'Sullivan would have.
The Red Sox will use David Price, Steven Wright, O'Sullivan, Rick Porcello and Price again in the five games before the break. The club has an off-day Thursday.
• The plan for Joe Kelly's transition to the bullpen is for the righty to pitch for Double-A Portland in a game at Manchester, N.H., on Wednesday and Class A Short Season Lowell on Saturday. Kelly would likely move to Triple-A Pawtucket after the All-Star break, though the Red Sox could recall him by then if they need him.
• Blake Swihart has made progress of late in his rehab from a severe left ankle sprain, and Farrell said he is a little ahead of schedule. Still, he isn't close to a return to action. The switch-hitting catcher/left fielder has taken swings left-handed, but his ankle isn't ready for the impact from the right side.
"Today was the first day to just begin some easy work in the outfield. He's not ready for a lot of change in direction," Farrell said. "This was more just to see the ball in the air and start to get the feel of the ground underneath him. You spend the amount of time that he has in a cast and a boot, this is a very slow process early on. Even though we feel like he's ahead of schedule with us, there are still some baby steps we've got to go through, and that's the first of it today."