$\Omega$ is black square

[sr71684]

When using $\Omega$ I get a black square, anyone know a pro tip to fix this?

Eg:

\pi(\bf{o}_t|\bf{a}_{t-1})_\text{RPM} \rightarrow \{\Omega_1, \Omega_x, \Omega_y, \Omega_z\}

yields:

I have tried:

\usepackage{amsmath}
\usepackage{amssymb}