Answer:
I'm not quite sure how to explain, but it really just depends on the sentence. Example: I think one (of the) girls has a secret to share. or Would you like one (of) those?
Explanation: It's basically like this. Most, if not all of the time when you use the word (of) ... there will be a these, those, the, their, or them, after the word... I really really hope this makes sense :)
Also, in some cases there will be an (a) after the word of...
