Agyei, 22, has joined the U's for an undisclosed fee after agreeing a three-year deal.
Former MK Dons boss Robinson, 38, said there were "a couple more" new signings to come after his own new deal at the Kassam Stadium was confirmed.
Robinson joined Oxford in March 2018 and led them to 12th place last season.
"Fans saw the way we ended last season and the way we want to keep building on the pitch," said Robinson.
"But you look around at the training ground, the academy, the whole club and we are starting to put a great infrastructure into place."