Roy scored 17 goals and added 27 assists in 80 games with the Sabres in 2011-12. His best season was in 2007-08, when he had 32 goals and 49 assists.
Ott, a known agitator, had 11 goals, 28 assists and 156 penalty minutes in 74 games with the Stars last season.
Pardy, a six-foot-four, 220-pound blue-liner, had three assists and 16 penalty minutes in 36 games with Dallas.
Roy will be counted on to replace some of the offence Dallas lost when it traded Mike Ribiero to the Washington Capitals at the 2012 NHL Draft.