Replacing the gaussian semigroup in the heat kernel estimates by the Ornstein-Uhlenbeck semigroup on ${ℝ}^d$, we define the notion of Kolmogorov kernel estimates. This allows us to show that under Dirichlet boundary conditions Ornstein-Uhlenbeck operators are generators of consistent, positive, (quasi-) contractive $C_0$-semigroups on $L^p\left(\Omega \right)$ for all $1\le p\<\infty$ and for every domain $\Omega \subseteq {ℝ}^d$. For exterior domains with sufficiently smooth boundary a result on the location of the spectrum of these operators is also given.