LaTeX Error: Option clash for package xcolor

The beamer class loads xcolor per default. You can issue the option via a beamer-option

\documentclass[10pt,xcolor={usenames}]{beamer}

olivegreen on the other hand needs the dvipsnames option and is called OliveGreen:

\documentclass[10pt,xcolor={dvipsnames}]{beamer}
\setbeamertemplate{navigation symbols}{}
\usepackage{caption}
\usepackage{subcaption}
\usepackage{url}

\author{The author}
\title{Package conflicts}
\date{October 3rd, 2012}

\begin{document}

This is supposed to be \textcolor{OliveGreen}{olivegreen}
\end{document}

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You could do

\documentclass[10pt,xcolor={usenames,dvipsnames}]{beamer}

but don't (usenames is obsolete).
TeXLive 2009 is old …

that is a problem with beamer which should pass the package option not in this way. You can use

\PassOptionsToPackage{dvipsnames}{xcolor}
\documentclass[10pt]{beamer}
[...]

The same problem occurs if you want to load options for package graphicx. Then you also have to use the macro \PassOptionsToPackage