O'Sullivan, 26, posted a 3.96 ERA in 25 innings (three starts, four relief appearances) for the Padres last season. However, the former Angel and Royal also walked more batters (14) than he struck out (12) in that time. O'Sullivan has struggled overall at the Major League level, pitching to a 5.89 ERA with 4.2 K/9 and 3.5 BB/9 in 218 2/3 innings. The former third-round pick has a 4.40 ERA with 6.1 K/9 and 2.7 BB/9 in 486 1/3 career innings at Triple-A.
Sappelt, 27 in January, batted .240/.269/.280 in 78 plate appearances for the Cubs in 2013. One of three players acquired by Chicago in the trade that sent Sean Marshall to the Reds (the others were Travis Wood and Ronald Torreyes), Sappelt is a career .251/.301/.343 hitter in 274 big league plate appearances. He's been much better at Triple-A, batting .279/.332/.402 in 1356 PAs.
Gwynn spent the bulk of the 2013 campaign with the Dodgers' Triple-A affiliate, where he batted .300/.393/.384 with a pair of homers and 12 steals in 391 plate appearances. The 31-year-old is an outstanding defender in center field, making him a nice depth piece for the Phils.
Marson, 27, was drafted by the Phillies in the fourth round of the 2004 draft. Philly traded him to the Indians along with Jason Knapp (who is attempting a comeback), Carlos Carrasco and Jason Donald in the 2008 blockbuster that netted Cliff Lee. Marson spent most of the 2013 season on the disabled list with right shoulder inflammation. Yan Gomes' breakout with the Indians displaced Marson as the team's backup catcher. He is a .219/.309/.299 career hitter in 882 Major League PAs.