The 32-year-old former Malmo left-back has Champions League experience with the Swedish club and was most recently with Azerbaijan club Gabala FK.
"I've always had a dream to play in England," he told BBC Radio Oxford. "I'm really looking forward to enjoying the atmosphere at this club."
Ricardinho has the option to extend his Oxford deal by a further year and is available to face Oldham on Saturday.
Find all the latest football transfers on our dedicated page.
Oxford United have signed Brazilian defender Ricardinho on a one-year deal.