How draw the angle theta?

Question

I have advanced this (image of left), but not know how draw the angle theta.

Help please.

\documentclass[circuitikz,border=5pt]{standalone}
\usepackage[latin1]{inputenc}

\usepackage{tikz}
\usepackage{tikz-3dplot}
\usepackage{listings}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}

\begin{document}

\tdplotsetmaincoords{70}{120}

\begin{tikzpicture}[
    scale = 1.2,
    tdplot_main_coords
]

    \node[left] at (0,0,0.7) {{\scriptsize $O$}};

    \begin{axis} [
        xmin = 0, xmax = 6,
        ymin = 0, ymax = 6,
        zmin = 0, zmax = 15,
        axis lines = middle,
        ticks = none,
        xlabel  = {\scriptsize $x$},
        ylabel  = {\scriptsize $y$},
        zlabel  = {\scriptsize $z$}
    ]

    \coordinate (O) at (0,0,0);
    \coordinate (P) at (3,3,3);

    \node[fill,circle,inner sep=0.5pt] at (P) {};   
    \draw[] (0,0,0) -- (P) node[anchor = south]{{\scriptsize $P \, (x, y, z)$}};

    \draw[] (P) -- (3,3,0);
    \node[] at (3.2,3,1.5) {{\scriptsize $z$}}; 

    \draw[dashed] (3,3,0) -- (3,0,0);
    \draw[dashed] (3,3,0) -- (0,3,0);

    \draw[] (O) -- (3,3,0);
    \node[] at (2,1.4,0) {{\scriptsize $\bar{r}$}};

    \end{axis}

\end{tikzpicture}

\end{document}