The oxygen abundance in the solar photosphere, and consequently the solar metallicity itself, is still a controversial question with far-reaching implications in many areas of astrophysics. This paper presents a new determination obtained by fitting the forbidden O i line at 6300 Å with an observational 3D model. The approach presented here is novel because previous determinations were based either on 1D empirical stratifications or on 3D theoretical models. The resulting best-fit abundances are log ɛ(O) = 8.90 and log ɛ(Ni) = 6.15. Nevertheless, by introducing minor tweaks in the model and the procedure, it is possible to retrieve very different values, even down to log ɛ(O) = 8.70. This extreme sensitivity of the abundance to possible systematic effects is not specific to this particular work, but probably reflects the real uncertainty inherent to all abundance determinations based on a prescribed model atmosphere. As with the earlier version, the new 3D model is publicly available and may be downloaded from the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/577/A25