Based on high quality observations of multiplet V1 of O II and the NLTE atomic computations for O II we study the density and temperature of a sample of PNe. We find that, in general, the densities derived from recombination lines of O II are similar to the densities derived from forbidden lines. This implies that the signature for oxygen rich clumps of high density and low temperature is absent in most of the objects of our sample. Electron pressures derived from the hotter zones are similar or slightly larger than those derived from the colder zones, suggesting the presence of shock waves. The average temperatures and t^{2} values derived from H, He and O lines are similar and consistent with chemical homogeneity. These results suggest that the abundances of these objects are the ones derived from recombination lines.