Please help me to comprehend the mean of preposition "off" in the sentence below

I get this sentence:

In the film, Freddie Mercury threatens to walk out on the label if the record executive played by Myers refuses to release "Hobemian Rhapsody" as the first single off its album "A night at opera".

I take "off" (after "single") as Preposition, is it right?

From my comprehension, its meaning seemed to be affiliated. 

I mean it seemed like that "the first single of its album".

Oh no, I'm totally muddled.