Extension domains for Hardy spaces

We show that a proper open subset $\mathrm{\Omega }\subset {\mathbb{R}}^n$ is an extension domain for $H^p$ ($0<p\le 1$), if and only if it satisfies a certain geometric condition. When $n(\frac{1}{p}-1)\in \mathbb{N}$ this condition is equivalent to the global Markov condition for ${\mathrm{\Omega }}^c$, for $p=1$ it is stronger, and when $n(\frac{1}{p}-1)\mathrm{\notin }\mathbb{N}\cup \{0\}$ every proper open subset is an extension domain for $H^p$. It is shown that in each case a linear extension operator exists. We apply our results to study some complemented subspaces of $BMO({\mathbb{R}}^n)$.