The 20-year-old has joined the Third Division outfit in a six-week loan deal, and links up with another former Hammer in the shape of manager Paul Brush.
The defender has recovered from a knee injury which curtailed his progress at Upton Park.
"Having watched Ezy play a number of times, I'm delighted to be able to get him," Brush told Orient's official website.
"He's very determined to be a league player and he's got the kind of attitude we want.
"He's a strong player who will give us competition for places in defence, and compliments the players we already have.
"On the pitch, he's known as being a vocal player too, and West Ham have allowed him to play in the FA Cup for us, which was important."
Iriekpen made a superb debut on Tuesday when he scored for The O's in a 3-2 home victory over Division Two side Peterborough United in the Southern Section first round of the LDV Vans Trophy.
