Two metal plates carrying charges Q and -Q face each other at some...

The electric field between two oppositely charged parallel plates is given by $E = \dfrac{Q}{A \epsilon_0 K}$, where $Q$ is the magnitude of charge on the plates, $A$ is the area of the plates, and $K$ is the dielectric constant of the medium between them.

When the plates are immersed in oil, the dielectric constant of the medium is $K > 1$.

When the oil is drained out, the medium between the plates becomes air or vacuum, for which the dielectric constant is $K = 1$.

Since the charge $Q$ on the isolated plates remains constant and the dielectric constant $K$ decreases, the electric field $E$ between the plates will increase.