In big-o notation, when we say that a function $f$ is $O(x^2)$ we're basically saying that
$$|f(x)|\le M |x^2|$$ for some constant $M>0$ and for all $x>x_0$
for some $x_0$.
But $O$ is under-specified without a statement such as $x\to\infty$ or $x\to 0$.
For example: "$x^2=O(x)$ as $x\to 0$" is true; but "$x^2=O(x)$ as $x\to \infty$" is false.
In some areas, like computer science, people tend to consider this situation exclusively, so "$x\to\infty$", is understood without saying.
If we are talking about $x\to 0$, which is a different situation; we are considering $x$ near $0$, not large $x$.
- $f(x) = O(x)$ as $x\to 0$ means: there is $C$ such that $|f(x)|\le C|x|$ for all $x$ sufficiently close to $0$. Such a function might look like this:
That is, its graph is contained in some bowtie-shaped region like the one between the red lines.