Oni vs. One
What's the difference between oni and one? When do you use which one?
In the Basics 2 skill notes :
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.
It is not really addressed in the tips and notes, but the key word is really that oni has to include a male "person." Other masculine-gendered nouns that are not people (e.g. animals and objects) would be included in one (unless there is at least one male person in the group).