How to pluralize an acronym which ends in 'S' correctly?

To define plural exceptions, you can use \acrodefplural:

\documentclass{article}
\usepackage{acronym}

\acrodefplural{OS}[OS's]{Operating Systems}

\begin{document}

\acp{OS}


\begin{acronym}
\acro{OS}{Operating System}
\end{acronym}

\end{document}

More information can be found on Section 2.3.1 Non standard and foreign plural forms of the acronym package documentation (see here).

To also have the plural show up as a link to the acronym list, enter two acronym entries similar to as such:

\acro{TSE}{transmissible spongiform encephalopathy}
\acrodefplural{TSE}{transmissible spongiform encephalopathies}

Then make sure you reference the acronym in the text body as such:

\acp{TSE}

This will now enter in the text with a link to the acronyms list as such:

"... transmissible spongiform encephalopathies (TSEs) ..."