Det er ost = It is Original Soundtrack :P
how do you say then "there is cheese"?
Like this, or "Det [fins/finnes] ost" (There exists cheese).
How do you say "This is cheese."?
I think that would be "Dette er ost"
Why is there no en or et? For example: Det er en ost.
Because it's not referring to a specific cheese, but to cheese in general. "Ost" is used as a collective noun in this sentence.
Why "It is a cheese" wrong?
Cheese is a collective noun and doesn't require an indefinite article unless you want to include an adjective, e.g., "It is a Norwegian cheese," (Det er en norsk ost.)