  In this article we introduce the Gaussian Sobolev space $W^{1,2}(\mathscr
O,\gamma)$, where $\mathscr O$ is an arbitrary open set of a separable Banach
space $E$ endowed with a nondegenerate centered Gaussian measure $\gamma$.
Moreover, we investigate the semimartingale structure of the infinite
dimensional reflecting Ornstein-Uhlenbeck process for open sets of the form
$\mathscr O=\{x\in E\, :\, G(x)<0\}$, where $ G$ is some Borel function on $E$.
