Just minutes after Colin Lee announced that he was set to make his mind up about Oster, the midfielder took the decision out of his hands and joined The Royals.
Talking to Reading's official site, the former Welsh international explained his decision.
"I was training with Millwall for the last week and a half and I got a call from Kevin Dillon (of Reading) yesterday morning.
"Millwall had offered me something, talks were ongoing but as soon as Reading came on the phone I had no hesitation."