In formal French, on means an indeterminate someone (singular or plural). In colloquial French, on can either have this meaning or can be a replacement for the first person plural nous. The verb form doesn't change based on the meaning.
On m'a maté le cul.
On a maté mon cul.
(Note spelling: “on a”.) These sentences are synonyms, meaning “someone checked out my ass” or “people checked out my ass”. The meaning excludes the first person interpretation.
In French, when talking about body parts, it is common to use a personal pronoun as a verb complement and a determinate article on the body part, where you would instead logically expect a possessive adjective on the body part. The use of a personal pronoun gives the statement a personal feel: the self is affected through a part of the body. The use of a possessive adjective puts some distance between the self and the body part. Depending on the meaning, the two expressions may be synonyms, or they may have different connotations, one of them may be prefered or mandatory.
J'ai mal à la tête. = My head hurts.
[I hurt, and my head is the part of me that causes the hurt.]
On voit ma tête sur la photo. = You can see my head on the photo.
[The image on the photo depicts my head. It's not me, it's a representation of a part of my body.]
Je me suis coupé le doigt. = I cut my finger.
[This affects me, so the personal pronoun is strongly prefered.]
J'ai le doigt en sang. / Mon doigt est en sang. = My finger is bleeding heavily.
[Here I'm looking at my finger rather than feeling it directly so there is enough detachment to permit both expressions.]
Switching to the third person brings in a different difficulty.
On s'est maté le cul.
The personal pronoun se (here elided to s') is a reflexive pronoun. Here on has to be the first person plural (the vulgar words make it impossible to interpret on in the formal way). The sentence can mean either “we checked out our own asses” or “we check out each other's ass”.
On a maté son cul.
On lui a maté le cul.
These two sentences mean “We check out his/her ass.” Lui is the third person singular personal pronoun; because it is before the verb, the same form can be either masculine or feminine (otherwise the feminine form would be elle).
On m'a volé mon portable.
Quelqu’un m'a volé mon portable.
Quelqu’un a volé mon portable.
These sentences have exactly the same meaning. The first one is more natural. The third one puts a tiny bit of emphasis on the fact that it's someone in particular but the speaker doesn't know who. The second sentence is formally incorrect but is fairly common in informal speech; the redundant reference to the speaker shows the speaker's involvement in the event.