I know a few people like Omid. They're just overly eccentric, at times, and people give them funny looks. I don't really dislike Omid, per say, I just don't like his timing. We just got screwed over ten ways from Sunday, and now we happen to run across this happy-go-lucky guy eyeballing Clemmy. If anything, he is an annoyance, at this point and time.
I think of them as, replacement-wise:
Christa = Lily.
Omid = Carley/Doug.
Charles = Mark.
I just have a feeling they'll die in the opposite order, if at all. Two episodes isn't really enough to introduce us to Christa and Omid as replacement characters and they'll probably die because of it (We're really not used to them, yet). Charles will probably stay around, as a filler, if anything. I think he replaces Mark, just with a different personality basis. He'll be useful, now and again, and just tag along.
Really, only time will tell how these new characters pan out.