  We present ten high-precision light curves of four transits in the XO-1
planetary system, obtained using $u$, $g$, $r$, redshifted H$\alpha$, $I$ and
$z$ filters. We use these to measure the physical properties, orbital
ephemeris, and a transmission spectrum of the planet covering the full optical
wavelength range. We augment this with published HST/WFC3 observations to
construct a transmission spectrum of the planet covering 0.37 to 1.65 $\mu$m.
Our best-fitting model to this spectrum is for a H$_2$/He-rich atmosphere
containing water (3.05$\sigma$ confidence), nitrogen-bearing molecules NH$_3$
and HCN (1.5$\sigma$) and patchy cloud (1.3$\sigma$). We find that adding the
optical to the near-infrared data does not lead to more precise constraints on
the planetary atmosphere in this case. We conduct a detailed investigation into
the effect of stellar limb darkening on our results, concluding that the choice
of limb darkening law and coefficients is unimportant; such conclusions may not
hold for other systems so should be reassessed for all high-quality datasets.
The planet radius we measure in the $g$-band is anomalously low and should be
investigated with future observations at a higher spectral resolution. From the
measured times of transit we determine an improved orbital ephemeris, calculate
a lower limit on the modified stellar tidal quality factor of
$Q_\star^{\,\prime} > 10^{5.6}$, and rule out a previously postulated
sinusoidal variation in the transit times.
