hey- great question. i've asked this to at least 2 rad onc chairs who say that unless your either the first author or presenter, you typically omit all other abstracts from your CV. they felt it looked like padding since you'll ideally have a full corresponding manuscript at some point, which you will put on CV regardless of where you are listed as author.
ultimately it's your decision. in your case, it's an oral presentation but really an "oral" for the presenter. i'd probably omit.