I went to oxford street on 20th of december one year and it was horrible! Sooo busy it was unreal an you couldn't actually g in to any shops to buy stuff but earlier in december on a weekday should be good. oxford street is always busy tho!
You taking kiddies with you aly?
If i were you, i'd get to oxford street early (when shops open) and get out before lunch time to avoid the crowds. Knightbridge is generally quieter, but still busy. But its all high end stuff so if your like me its more for browsing and dreaming rather than buying!
I personally love camden market cos its unusual and a lil bit dodgy LOL. But its also intimidating if ur not in to that sort of thing. x