Uncovering lines in an equation in split environment

Question

How does one uncover lines in a multi-line equation one by one? I have taken a careful look at the answers provided here. It hasn't solved my problem. For example, my equation is the following and I want to uncover the two lines one at a time. Any help would be greatly appreciated.

\documentclass{beamer}
\usepackage{amsmath, amsfonts, amssymb, cancel}
\begin{document}
\begin{frame}
\frametitle{Steady State Relationship between $O^m, O^a$}
\begin{equation}
\begin{split}
(\rho +\sigma\bar\omega^m)O^m&=O^a-\phi L^a-\phi L^r-\eta O^r\\
\frac{O^m}{O^a}&=\frac{1-\phi l^a-(\phi l^r+\eta) O^r/O^a}{\rho +\sigma\bar\omega^m}
\end{split}
\end{equation}
\end{frame}
\end{document}

Answer 1

The following inserts the math content using \alt. \phantom is used to maintain the appropriate spacing on slides where the content is not needed. All of this is contained within a macro \disponslide{<overlay>}{<stuff>}, which prints <stuff> using the overlay specification <overlay>, and \phantom{<stuff>} outside of <overlay>:

\documentclass{beamer}% http://ctan.org/pkg/beamer
\let\Tiny\tiny% http://tex.stackexchange.com/q/58087/5764
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\newcommand{\disponslide}[2]{%
  \alt<#1>{#2}{\phantom{#2}}}
\begin{document}
\begin{frame}
  \frametitle{Steady State Relationship between $O^m, O^a$}
  \begin{align}
    (\rho +\sigma\bar\omega^m)O^m    &= O^a-\phi L^a-\phi L^r-\eta O^r \\
    \disponslide{2}{\frac{O^m}{O^a}} & \disponslide{2}{{}=\frac{1-\phi l^a-(\phi l^r+\eta) O^r/O^a}{\rho +\sigma\bar\omega^m}}\only<1>{\notag}
  \end{align}
\end{frame}
\end{document}

You have to split the overlay between the alignment positions &.

Answer 2

The overlay specifications do not work inside amsmath environments, including split. Indeed the beamer documentation says:

\pause[⟨number ⟩] This command causes the text following it to be shown only from the next slide on, or, if the optional ⟨number⟩ is given, from the slide with the number ⟨number⟩. If the optional ⟨number⟩ is given, the counter beamerpauses is set to this number. This command uses the \onslide command, internally. This command does not work inside amsmath environments like align, since these do really wicked things.

Instead I suggest you display this as two separate equations - in the given example the alignment is not essential.

If such displays are necessary you could consider the equationarray environment from eqnarray:

\documentclass{beamer}

\usepackage{eqnarray,amsmath, amsfonts, amssymb, cancel}

\begin{document}

\begin{frame}
\frametitle{Steady State Relationship between $O^m, O^a$}
\begin{equationarray}{r@{}l}
    (\rho +\sigma\bar\omega^m)O^m&{}=O^a-\phi L^a-\phi L^r-\eta
    O^r\pause \\
    \frac{O^m}{O^a}&{}=\frac{1-\phi l^a-(\phi l^r+\eta) O^r/O^a}{\rho
    +\sigma\bar\omega^m}
  \end{equationarray}
\end{frame}

\end{document}

As you see, will have to take more care with spacing near alignment points. For example, writing &{}= instead of &=, above.