The confusing Unix permissions terminology
You could point out that a file has an owning user and an owning group. Since “owner” appears twice, specifying o for “owner” would be ambiguous. That's weak because the owning group doesn't really have any special privileges on the file, unlike the owning user: the owning user can change the file's permissions and other metadata, but the owning group only has the privileges granted by the file's permissions.

I think the best approach is to pair up user and group so that people remember them together. There's user and group: u and g. There are basic file permissions for one user and for one group. ACL entries can apply to either a user or a group. The command ls -l has two columns for ownership, one with the user and one with the group. Beware that sometimes the naming is not consistent. On the system there's a user database (/etc/passwd) and a group database (/etc/group). The chown command takes both a user and a group, but there's also a separate chgrp command and so chown is often used for the user only, which tends to reinforce the association between “user” and “owner”, so avoid mentioning that if you're trying to dispel the association between “user” and “owner”.