  Detecting heavy atoms in the inflated atmospheres of giant exoplanets that
orbit close to their parent stars is a key factor for understanding their bulk
composition, and the processes that drive their expansion and interaction with
the impinging stellar wind. Here, we use archive data obtained with the Cosmic
Origins Spectrograph onboard the Hubble Space Telescope to report an absorption
of ~6.4+/-1.8% by neutral oxygen during the HD 189733b transit. Scaling
published HI results from a simple hydrodynamic model of HD 189733b, a vertical
OI column density of ~8x10^15/cm2 produces only a 3.5% attenuation, implying
that non-thermal line broadening or super-solar abundances are required. We
also report evidence of short-time variability in the measured stellar flux, a
variability that we analyze and compare to solar flaring activity. In that
frame, we find that non-statistical uncertainties in the measured fluxes are
not negligible, which calls for caution when reporting transit absorptions.
Despite these uncertainties, we also show a possible detection for both a
transit and early-ingress absorption in the ion CII 133.5nm lines. If
confirmed, this would be the second exoplanet for which an early-ingress
absorption is reported. Assuming the HD 189733b magnetosphere to be at the
origin of the early absorption, we use the Parker model for the stellar wind
and a particle-in-cell code for the magnetosphere to show that its orientation
should be deflected ~10-30{\deg} from the planet-star line, while its nose's
position should be at least ~16.7 Rp upstream of the exoplanet in order to fit
the CII transit light curve. The derived stand-off distance is consistent with
a surface magnetic field strength of ~5.3 Gauss for the exoplanet, and a
supersonic stellar wind impinging at ~250 km/s, with a temperature of 1.2x10^5K
and a density ~6.3x10^6/cm^3 at the planetary orbit, yet the fit is not unique.
