convergence in operator norm Let $H$ be Hilbert space, $\{a_n\}_{n=1},$ be ONS and $K$ be a compact operator. Suppose $K_nx:=(Kx,a_n)a_n$, $\sum _{n=1}^{N} K_n$ convergent as $N \to \infty$.
So,$||K_nx||=|(Kx,a_n)|=|(x,K^* a_n)| \leq ||x|| ||K^* a_n||$ and so $\lim ||K_n||\leq \lim ||K^* a_n||=0$ since $K^*$ is compact and $\{a_n\}$ weakly convergent.

Let $L : H\to H$ be given by 
$$ L(x) = \sum_{n=1}^\infty \langle x, a_n\rangle b_n.$$
Then 
$$K_N :=\sum_{n=1}^N K_n  =L_N \circ K,$$
where $L_n (y)= \sum_{n=1}^N \langle y, a_n \rangle b_n.$ To show that $K_N$ converges, it suffices to show that $L^{-1}\circ K_N$ converges to $K$.But 
$$L^{-1} \circ K_N (x)= \sum_{n=1}^N \langle  Kx, a_n\rangle a_n$$ 
and it reduces to the usual argument where one proves that finite rank operators in Hilbert space are dense in the space of compact operators, that can be found here. 
