Robson made 15 appearances for Oyonnax in the Top 14 last season after joining the French side in February 2015.
The 30-year-old had previously spent his entire career at Harlequins, where he made 146 Premiership appearances.
"I'm looking forward to the challenge of trying to win promotion back to the Premiership," he told the London Irish website.
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Championship club London Irish have signed Oyonnax and former Harlequins lock George Robson on a one-year deal.