  Let $G$ be a non-compact simple Lie group with Lie algebra $\mathfrak{g}$. Denote with $m(\mathfrak{g})$ the dimension of the smallest non-trivial $\mathfrak{g}$-module with an invariant non-degenerate symmetric bilinear form. For an irreducible finite volume pseudo-Riemannian analytic manifold $M$ it is observed that $\dim(M) \geq \dim(G) + m(\mathfrak{g})$ when $M$ admits an isometric $G$-action with a dense orbit. The Main Theorem considers the case $G = \widetilde{\mathrm{SO}}_0(p,q)$ providing an explicit description of $M$ when the bound is achieved. In such case, $M$ is (up to a finite covering) the quotient by a lattice of either $\widetilde{\mathrm{SO}}_0(p+1,q)$ or $\widetilde{\mathrm{SO}}_0(p,q+1)$. 
On low-dimensional manifolds with isometric $\mathrm{SO}_0(p,q)$-actions