[The Wikipedia page for Big-$O$](https://en.wikipedia.org/wiki/Big_O_notation#Formal_definition) states that we write $f(x)=O(g(x))$ as $x\rightarrow\infty$ iff
$$\limsup\_{x\rightarrow\infty}{\left|\frac{f(x)}{g(x)}\right|}<\infty.$$
But does this not imply that $f$ is $O$ of multiple functions? For instance, $f(x)=2x^3+x$ is $O(x^3)$ since $\limsup\_{x\rightarrow\infty}{\left|\frac{2x^3+x}{x^3}\right|}=2<\infty$. But we must also have that $f$ is $O(e^x)$, since $\limsup\_{x\rightarrow\infty}{\left|\frac{2x^3+x}{e^x}\right|}=0<\infty$. Am I missing something here?