Marson appeared in eight games with the Phillies before the trade.
Gwynn, 31, is a player who has interested the Phillies in the past. He is the son of Hall of Famer Tony Gwynn, but his major-league success has been fleeting. He spent all of 2013 in the minors with Los Angeles. He has a career .630 OPS in 605 major-league games.
Sappelt, 26, appeared for the Cubs in each of the last three seasons and can play all three outfield positions. He is a career .251 hitter with a .644 OPS. O'Sullivan, 26, has logged 50 games (37 starts) in the majors with a 5.89 ERA. He could assume a spot in the IronPigs' rotation.
The Phillies promoted John Mizerock to serve as their assistant hitting coach next season. Mizerock, 53, was a coach with single-A Clearwater for the last three seasons.
Phillies general manager Ruben Amaro Jr. said he wanted to hire a lefthanded hitter for another perspective. Mizerock replaces Wally Joyner, who left to become Detroit's hitting coach. The Punxsutawney, Pa., native spent five years in the majors as a coach with Kansas City, the most recent in 2009.
Charlie Manuel is expected to accept a role in the Phillies front office that would involve some minor-league hitting instruction, scouting and public relations. Amaro said he is still talking about details with Manuel.