Memphis hasn't looked back since leaving United in January.
He declared: "Leaving Manchester? It was not very complicated. I'm young, I need to play.
"Looking back at the last time I played 90 minutes before arriving here, it was a year ago. It's incredible!"
OL coach Bruno Genesio also said of the Dutchman: "He brings a lot of joy, carefree. He quickly became integrated by his joy of playing football. It is a good thing for the squad."