Oni and one
Oni and one are both translated as they. However, they are used in different contexts.
Oni is used to refer to all groups that contain at least one male person. In other words, both all-male and mixed male and female groups are referred to as oni.
One is used to refer to all groups that do not contain any male persons. Therefore, groups of female persons, groups of neuter persons and groups containing both female and neuter persons are referred to as one.
You always use “one” when referring to groups of children, animals or objects. However, if “oni” is used, it must denote a group of people.