\begin{table}%T3
%\centering
\par
\caption{Oxygen abundance determinations. Diagnostics have been taken from
  following papers: (1) \citet{pettini04}, (2) \citet{denicolo02}, (3)
  \citet{kobulnicky99}.}
\label{tab:o_abun}
\begin{tabular}{ll}
\hline \hline
\smallskip
Diagnostic     & $12+\log$(O/H) \\
\hline
O3N2 (1)     & $8.66\pm0.10$ \\
N2   (1)     & $8.65\pm0.03$ \\
N2   (2)     & $8.77\pm0.06$ \\
$R_{23}$ (3)   &  $9.02\pm0.13$ \\
\hline
\end{tabular}
\end{table}