Big-O Justification Question
"The Big-O notation describes the limiting behavior of a function when
  the argument tends towards a particular value or infinity, usually in
  terms of simpler functions."-Wikipedia


If you see the formal definition, then the only restriction is that the domain of functions must be $\mathbb{R}$. Hence, by definition the range of the functions can at least be any vector space.
In the most common use in CS, the range of the functions is also taken to be $\mathbb{R}^+$. 

To answer your question, by definition, the left side can be any real positive value.