In exchange for Hemsky, who has spent his entire 11-year career in Edmonton, the Oilers received a third-round draft selection in 2015 and a fifth-round pick in 2014.
"I really like [Edmonton], I love the guys over here and I like the organization," Hemsky said. "I need a change. It's a good challenge for me, I've been here for so long in this situation, so I'm happy for the challenge."
The Oilers also will be responsible for half of Hemsky's remaining salary for this season, a source told ESPN.com's Pierre LeBrun. Hemsky was scheduled to make $5.5 million this season.
Hemsky, 30, has nine goals and 17 assists this season.
The Hemsky deal was one of two transactions made Wednesday by the Senators, who also signed veteran defenseman Chris Phillips to a two-year extension worth $5 million.
ESPN.com's Pierre LeBrun contributed to this report.