Overlining, underlining and bold characters

$\underline{one} \overline{two}$

produces $\underline{one} \overline{two}$. This is not a useful facility if it's used more than once on a line. The lines are produced so that they don't quite overlap the text; lines over or under different words won't in general be at the same height.

To be able to reproduce bold maths, it's best to use the bm package. $E = \bm{mc^2}$ produces E = mc².

Alternatively, you can use \mathbf{} to create bold characters - $\mathbf{F}_2^3$ produces F₂³. or you can use the following idea

\usepackage{amsbsy} % This loads amstext too
\begin{document}
$\omega + \boldsymbol{\omega}$

% Use the following if whole expressions need to be in bold
{\boldmath $\omega $}
\end{document}