Sure, having a wise, old mentor is cool, but you know what's really cool? Having two wise, old mentors.
Obi-Wan and Yoda share the same role in Return of the Jedi: They give Luke the brain boost he needs to confront Darth Vader. Both insist that the only way Luke can triumph—and become a full-on Jedi—is to face his father:
YODA: No more training do you require. Already know you, that which you need.
LUKE: Then I am a Jedi.
YODA: Not yet. One thing remains: Vader. You must confront Vader. Then, only then, a Jedi will you be. And confront him you will.
Yoda puts a kinder spin on things, expressing his regret that Luke already rushed in and met Vader once before. Obi-Wan, meanwhile, indulges in a little reverse psychology, telling Luke that he thinks his father is probably too far gone to be redeemed, practically daring Luke to do what Obi-Wan himself couldn't and save Anakin.
Both mentors also urge Luke to check his gut. Yoda warns Luke that, if he lets his guard down and embraces the Dark Side, there will be no coming back. Obi-Wan, on the other hand, simply encourages Luke to bury his feelings deep if he wants to protect himself, and his plans, from the Emperor:
OBI-WAN: Your insight serves you well. Bury your feelings deep down, Luke. They do you credit, but they could be made to serve the Emperor.
While Obi-Wan and Yoda don't exactly play "Good Cop, Bad Cop" in the swamp with Luke, they do take different approaches to bucking young Skywalker up for battle. The effect? Luke leaves Dagobah wiser, secure in his faith, and ready to kick a little emotional and psychological butt.