I recently came across the following generalization of the Darboux-Weinstein lemma:
Let $N$ be a manifold endowed with two symplectic forms $\omega_1, \omega_2$, and let $P$ be a compact submanifold of $N$ along which $\omega_1 = \omega_2 + O_p(k)$, for some $k \in \mathbb{N}^* \cup \{+\infty\}$. Then there exists open neighbourhoods $U$ and $V$ of $P$ in $N$ and a diffeomorphism $f : U \rightarrow V$ such that $f = \text{id}_N + O_p(k+1)$ and $f^* \omega_2 = \omega_1$.
Here I do not know what $O_p(k)$ means, and I cannot find any references to it online. Any help would be much appreciated.