"Oni" is a group of people with at least one male person; "one" is a group of people, all of whom are not male (so could be women and/or children).
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Unlike Polish, and also Croatian, in Serbian "to need" (trebati & biti potreban) is not a regular verb as other verbs which require accusative.
I need a cat = Treba mi mačka
"Treba" is the 3rd person singular and it is the subject of the verb.
I need a cat = Trebam mačku
"Trebam" is the 1st person singular and it requires accusative - similarly as in English.