"oni" is used when you have a group containing at least one male person, "one" is used otherwise (groups of objects and groups of people with only women and children).
I heard there was ono as well, is it a group of its. I imagine it will be explained later on.
"a group of its" would be "one", as "one" is for not masculine-personal plural - ergo, everything that is not 'a group of people with at least one man'.
"ono" is exactly "it" in "he/she/it", but it's used rather rarely. You don't really use it for objects, and if you use it for a child... well, you usually either know the child's name (or at least gender), or just say "dziecko"...