A baseball team won and they said;
“We are the champions.”
Is this sentense OK?
Usually the champion is only one or one team.
Why is there -s here, can’t we just say
“We are the champion.” instead?
Because each team member wants the glory. 
Hello hime,
The sentence is correct. ‘Champions’ is plural because it’s a predicate nominative referring back to the plural subject ‘we’. Since a predicate nominate renames the subject, they must be equal in terms of number.
A similar issue would be to say “We are the winners”, or ‘We are the losers.’
If you had a singular subject, such as ‘He is the champion’, then you could use the singular form ‘champion’.
Even though the champion is just one team, there are many people (at least 9!) on the baseball team.
By the way, there’s a song by Queen which is called “we are the champions”
Maybe, you would fancy to listen to it. 
Heh, I was humming that while writing my reply.
I can’t decide which video to post, as they both feature some outrageous outfits, so what the heck, same song, different things to uhh…see.
black and white hairy chested tights version
village people outfit sponsored by Heinekin
But then, they aren’t called Queen for nothing. :lol:
So you never say “We are the champion.” always with -s?
I suppose you could use champion as an adjective, then you could say “We are the champion team”, but that’s different than how you asked.
No, you can’t say ‘We are the champion.’