The actors, along with Corey Feldman and Wil Wheaton, appeared in the coming of age adventure-drama and became close during their time on set.
O'Connell met up with his former movie star pals recently to mark the 25th anniversary of the film's 1986 release - but the get-together was tinged with sadness because Phoenix, who died of a drug overdose in 1993, wasn't there.
The actor tells Britain's OK! TV, "It was fun to see Wil Wheaton and Corey Feldman, and our director Rob Reiner, but it's also very sad because River Phoenix has passed away. There's a sadness to it."