Expressing that a function converges to 1 with linear rate using Landau notation
Both of the options you listed are acceptable. They have the same meaning; $f\in O(1/n)$ if and only if $1/f \in \Omega(n)$.

Let $f\in O(1/n)$. Then there exist $n_0,M&gt;0$ such that for all $n&gt;n_0$, $f\leq M/n$. Then $1/f\geq n/M$ for all $n&gt;n_0$, thus $1/f\in \Omega(n)$, since for $n&gt;n_0$, $1/f \geq 1/M \cdot n$.

The other direction is similar.