Your equation is wider than text width ... so you need to make shorter or locally enlarge horizontal space for it. If this still not gives satisfactory result, use of smaller fonts may help. In MWE below are considered both measures:
\documentclass{beamer}
\usepackage{calc}
\usepackage{changepage}
\makeatletter
\newsavebox\restorebox
\newenvironment{restoretext}%
{\@parboxrestore%
\begin{adjustwidth}{-8mm}{-8mm}%
\begin{lrbox}{\restorebox}%
\begin{minipage}{\linewidth}%
}{\end{minipage}\end{lrbox}
\usebox\restorebox
\end{adjustwidth}
}
\makeatother
\begin{document}
\begin{frame}{Conditional Hazard Ratio under Competing Risks}
\begin{itemize}
\item Competing Risks:
\item $ h(t) = h_1(t) + h_2(t) $
\item $ \theta_{k_1,k_2}(t_1,t_2) = \dfrac{h_{1,k_1}(t_1|T_2 = t_2, K_2 = k_2)}{h_{1,k_1}(t_1|T_2>t_2)} $
\begin{restoretext}\small
\begin{equation}
\dfrac{ \Pr(T_{1a}-T_{1b})(T_{2a}-T_{2b}) > 0 | (T_{1ab} = t_1, T_{2ab} = t_2), (K_{1ab},K_{2ab}) = (k_1,k_2)}{ \Pr(T_{1a}-T_{1b})(T_{2a}-T_{2b}) < 0 | (T_{1ab} = t_1, T_{2ab} = t_2), (K_{1ab},K_{2ab}) = (k_1,k_2)}
\end{equation}
\end{restoretext}
\item item
\item item
\end{itemize}
\end{frame}
\end{document}
By defined box restoretext
is first restored text width in list environment to \textwidth
and than with adjustwidth
macro from package changepage
enlarged for 8 mm on each side. With this and reducing font size to \small
the equation can be fitted into Beamer frame.