Minimal example (the problem is obvious): 

```
\documentclass[]{report}

\makeatletter
\def\@makechapterhead#1{
    \vspace*{-5.0em}
    {\parindent \z@  \normalfont
    \interlinepenalty\@M
    \LARGE{0\thechapter}
    \par\vspace{0.25cm}
    \flushleft\MakeUppercase{#1}}
    \par\vspace{3.5em}
}
\makeatother

\begin{document}

\chapter{First Chapter}
\setcounter{chapter}{9}
\chapter{Tenth Chapter}

\appendix
\chapter{Appendix A}

\end{document}

```

**Edit:** I forgot to mention something important. Only the displayed chapter numbers should have 0s in front of them, not definitions, theorems, etc. That is: Chapter 01, Definition 1.1., etc.