Technically yes, but quoting mathematical constants is definitely allowed. You could also say O(N) times for instance. The kind of language that is discouraged is writing "therefore efficiency increases by O(log(N)) times" as "∴ efficiency increases by O(log(N)) *".

For one it simply looks like you forgot to finish the formula.

I particularly dislike Greek letters, because I don't know how to pronounce them in my head. For "×" you at least know it's supposed to read "times".