Perron, 25, and Paajarvi, 22, each was a first-round pick.
Perron was taken No. 26 by the Blues in the 2007 NHL Draft. He scored 10 goals and finished with 25 points in 48 regular-season games last season, adding two assists in six Stanley Cup Playoff games.
"Right away you're in shock a little bit and don't know what to think, but I'm looking forward to the fresh start," Perron told the Oilers' website. "I'm excited to be joining such a young team with a lot of skill, and I think my game is going to fit in Edmonton. The passion the fans have for the game is something I'm really looking forward to. Hopefully I can come in and do my part."
Perron is a two-time 20-goal scorer who has career totals of 84 goals and 198 points in 340 regular-season games with the Blues.
"He's just got an incredible skill set, great set of hands" Oilers general manager Craig MacTavish told the team's website. "He's a pretty gritty player too as well. He'll go to the areas necessary to score goals. ...
"I feel like he's a really good fit for our team and the way that we play. There'll be more attack opportunities for him, more rush opportunities, and I think we potentially could be a better fit for his skill set than the team that he comes from."
MacTavish said he expects Perron to play left wing for new coach Dallas Eakins.
"This guy has, potentially, one of the best set of hands in the League," MacTavish told 630 CHED. "... I've talked before about being able to make a play in the defensive zone, to get the puck to Sam Gagner, or to get the puck to [Ryan] Nugent-Hopkins on the fly, and just cleaning up our breakouts, with our forwards having a greater skill set to make those direct plays. And we feel like he has this. ...
"He's a guy that can really shoot the puck as well. So I think that there's every reason to expect that we can improve upon his goal production. ... I think there's a good upside there."
Perron missed most of the 2010-11 season and the start of 2011-12 because of a concussion.
"There's always concern when someone's had one of those," MacTavish said. "But I'm less concerned because of the fact he's been healthy for a long period of time now."
Perron (6-foot, 205 pounds) said he does not shy away from contact.
"I'm going to try to add on to what they're trying to do, and put some points on the board," he said. "But I also want to create some havoc and draw some penalties. I'm no Milan Lucic, but I'm going to try to finish my checks every time I have the chance. I think forechecking is one of my best skills. It was something I did a lot in St. Louis, and I got good at it."
Paajarvi was selected at No. 10 by the Oilers in 2009. He scored nine goals and finished with 16 points in 42 games last season.
As a rookie in 2010-11, Paajarvi had career highs of 15 goals and 34 points in 80 games. He has career totals of 26 goals and 58 points in 163 regular-season games with the Oilers.
"Magnus is a well-rounded player who has good size and great skating ability," Blues general manager Doug Armstrong said. "He is just coming into his own and this will open up more ice time for Jaden [Schwartz] and Vladimir [Tarasenko]. I would like to thank David for his time in St. Louis and wish him well in Edmonton."