In speech English, people say “of them” so quickly that the ‘f’ sound cannot be heard. Then, how do you spell the contraction? “o’em” or “of’em”? I saw the former somewhere but the latter seems unfamiliar. What do you think?
More often it is the “th” sound that is lost; this casual pronunciation is represented by the spelling “of 'em” (two words). Writing “o’” for “of” is less common, but may be seen in some representations of dialect speech, for example. “o’ 'em” looks a bit much, but I suppose it is not impossible. I don’t recall ever seeing “o’em”. Do you remember where you saw it?