I did the same thing. I TRIED to save Omid first because his leg was screwed up, but the camera was giving me hell and I couldn't find the indicator on Omid. So I assumed I had to save Christa. Then when she chewed me out for not saving Omid first, I realized that I must have been able to do it after all.
I've been trying to do a "live with the consequences" run and this is the first time I reloaded. I feel kind of dirty about it, but since saving Omid first was my initial intent anyway, I don't think it's that big a deal.