The 37-year-old quit Charlton on Thursday morning after 16 months at the helm after growing frustrated with a lack of support from the club's absent owner.
Robinson has walked into a new job just hours later, though, having been taken on by fellow third-tier side Oxford on a two-and-a-half-year deal.
Chairman Sumrith Thanakarnjanasuth told the U's official website: "We are really pleased that Karl has agreed to join the club. He is one of the brightest young managers in the game; people forget that he is still only 37 because he has been manager in over 450 games already.
"He knows this league and the one above, he knows the players and I am sure he will do a great job for us. The Oxford fans have been very patient but I hope they agree that this is a very exciting appointment for the club."
Robinson, who has previously won promotion from League One with MK Dons, succeeds Pep Clotet in the Kassam Stadium dugout some two months after the Spaniard was sacked.