Showing that $f(x) \leq O(g(x))$.

Question

I have two (strictly) positive functions $f(x)$ and $g(x)$ defined on $\mathbb{R}$. I am required to show that $f(x) \leq O(g(x))$ for $x \to \pm \infty$. I showed that $f(x) \leq kg(x)$ for $|x| \to \infty$. Is this enough ?