The 31-year-old back rower arrives at Franklin's Gardens after a year at French outfit Bourgoin. Although Plymouth-born, Oakley has won international honours with Wales, and his three-year spell at Newport Gwent Dragons consolidates his considerable Aviva Premiership Rugby experience collected at Bristol and Leeds Carnegie.
Oakley has signed a full contract after impressing during a pre-season trial period which included eye-catching performances against Leinster, Moseley and Edinburgh, and he says he has enjoyed his time so far in the black, green and gold.
"It's great to be at a big club, certainly the biggest club that I've ever been at," he said. "I'm relishing the chance to do well here. Unfortunately Bourgoin's financial situation meant that things didn't go as well as I wanted in France, but Saints gave me a ring. Things went well and here I am!
"I think I'm coming to Northampton as a more rounded player from my time in France. I had to take a lot more onto my own back and become a lot more professional, rather than have it being forced on me, and I had to learn to communicate effectively in a different way.
"I'm glad I got away from the Premiership for a year; it was good to experience a different culture. So I feel like I'm here refreshed as a player and ready to make the most of my opportunity. The Saints' style of play suits my game and I hope I'll do well here."
Director of rugby Jim Mallinder says that Oakley will add to the depth in a crucial area in the Saints squad.
"Quality back rowers are always good to have at the club, and we believe that Rhys can contribute to the squad," he said. "Rhys has impressed us over the summer, both in training and in how he has played in the pre-season matches."