"on" can stand for "ovat" in spoken Finnish. The sentence decides its meaning.
"on" is also an ordinary written word. Check the meaning note beside each example; not every use is a shortening.
Spoken form of 'ovat': the plural takes the singular verb
Ne on töissä. - They're at work.
on here: are - spoken 'ovat': 'ne' takes the singular verb
Written Finnish: He ovat töissä.
Nää on tosi hyviä. - These are really good.
on here: are - spoken 'ovat': the plural takes the singular verb
Written Finnish: Nämä ovat todella hyviä.
Noi on liian kalliita. - Those are too expensive.
on here: are - spoken 'ovat': the plural takes the singular verb
Written Finnish: Nuo ovat liian kalliita.
Mikä sun nimi on? - What's your name?
on here: is - 3rd person of 'olla' (to be)
Written Finnish: Mikä sinun nimesi on?
Kato, toi on mun kaveri. - Look, that's my friend.
on here: is - 3rd person of 'olla' (to be)
Written Finnish: Katso, tuo on minun kaverini.
Varo, kahvi on tosi kuumaa. - Careful, the coffee is really hot.
on here: is
Written Finnish: Varo, kahvi on todella kuumaa.
Spoken and written Finnish are different registers. The examples show how "on" is used here and pair it with the written sentence. Learn the meaning in context rather than replacing every occurrence of a word.