Darder, 21, tallied six goals through 67 La Liga appearances at Malaga, and will wear No. 14 at Lyon.
"I wanted to come to OL since the beginning," Darder said. "It was a long and tough negotiation. I was nervous and a bit worried. Now that it's done, I can't wait to get started. OL showed interest in me and I know they're a big European club. The ideal club for me to continue my progression.
"It's always great to know that you're wanted and that people expect things of you; I really want to respond to those expectations as soon as possible. My profile? I don't compare myself to other players. The essential thing is to work hard. I want to make my own path. I'd like to thank Malaga for the two seasons I spent there."
Tousart, meanwhile, who is only 18 years old, will wear No. 29 at his new club.
"It was complicated," Tousart said. "I was flattered by the interest OL showed in me. Today I'm a very happy man; this is a great opportunity for me. The sporting project here is wonderful and long-term. I'll leave myself a bit of time to adapt, but after that I'll just need to put my head down and do whatever it takes to win a place in the team."