You can use the tikzlibrary "shapes" as shown here.
\documentclass[12pt]{article}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{shapes}
\begin{document}
%\tdplotsetmaincoords{60}{120}
\begin{tikzpicture}
\coordinate (O) at (0,0,0);
\coordinate (A) at (2,0,0);
\coordinate (B) at (0,2,0);
\coordinate (C) at (0,0,2);
% draw axis
\draw[-latex] (O) -- (A) node[below] {$y$};
\draw[-latex] (O) -- (B) node[above] {$z$};
\draw[-latex] (O) -- (C) node[below] {$x$};
\node[cylinder, draw, shape aspect=.5,
cylinder uses custom fill, cylinder end fill=green!50,
minimum height=1cm,
cylinder body fill=green!25, opacity=0.5,
scale=3, rotate=90]{};
\begin{scope}[shift={(4,0)}]
\coordinate (O) at (0,0,0);
\coordinate (A) at (2,0,0);
\coordinate (B) at (0,2,0);
\coordinate (C) at (0,0,3.5);
% draw axis
\draw[-latex] (O) -- (A) node[below] {$y$};
\draw[-latex] (O) -- (B) node[above] {$z$};
\draw[-latex] (O) -- (C) node[below] {$x$};
\node[cylinder, draw, shape aspect=.5,
cylinder uses custom fill, cylinder end fill=green!50,
minimum height=1cm,
cylinder body fill=green!25, opacity=0.5,
scale=3, rotate=-135]{};
\end{scope}
\begin{scope}[shift={(8.,0)}]
\coordinate (O) at (0,0,0);
\coordinate (A) at (2,0,0);
\coordinate (B) at (0,2,0);
\coordinate (C) at (0,0,2);
% draw axis
\draw[-latex] (O) -- (A) node[below] {$y$};
\draw[-latex] (O) -- (B) node[above] {$z$};
\draw[-latex] (O) -- (C) node[below] {$x$};
\node[cylinder, draw, shape aspect=.5,
cylinder uses custom fill, cylinder end fill=green!50,
minimum height=1cm,
cylinder body fill=green!25, opacity=0.5,
scale=3]{};
\end{scope}
\end{tikzpicture}
\end{document}

cylinder
shape.:)