  In this article, we study a number of properties of the K\"othe duals
$\mathcal{M}_{\varphi,w}$ of Orlicz-Lorentz spaces. An explicit description of
the order-continuous subspace of $\mathcal{M}_{\varphi,w}$ is provided.
Moreover, the separability of these spaces is characterized by the growth
condition $\Delta_2$. Consequently, the K\"othe dual space
$\mathcal{M}_{\varphi,w}$ has the Radon-Nikod\'ym property if and only if the
N-function at infinity $\varphi$ satisfies the appropriate
$\Delta_2$-condition. The comparison between $\mathcal{M}_{\varphi,w}$ spaces
is characterized via standard orders between Orlicz functions. As applications
of these results, we provide sufficient conditions for M-embedded
order-continuous subspaces of Orlicz-Lorentz spaces equipped with the Luxemburg
norm and prove the existence of a unique norm-preserving extension on
Orlicz-Lorentz spaces equipped with the Orlicz norm.
