Why is $\Omega$ used to denote the space of differential forms?

Question

A simple curious question about notation. The space of $k$-differential forms over a manifold $M$ is typically denoted by $\Omega^k(M)$. Does anybody know why, or where this notation originated? It would make sense to call them $A^k(M)$, for alternating, or $\Lambda^k(M)$, in reference to the wedge (I think some communities actually use these notations). Why the $\Omega$, then? Is there any record on where it was first used?