  We present new observations of O II recombination lines in ten bright
planetary nebulae, along with spatially-resolved measurements of O II and [O
III] in the Ring nebula NGC 6720, to study the discrepancy between abundances
derived from O II recombination lines and those derived from
collisionally-excited [O III]. We see a large range in the difference between O
II- and [O III] derived abundances, from no difference up to a factor six
difference. The size of this discrepancy is anti-correlated with nebular
surface brightness; compact, high-surface-brightness nebulae have the smallest
discrepancies. O II levels that are populated mainly by dielectronic
recombination give larger abundances than other levels. Finally, our long-slit
observation of the Ring nebula shows that the O II emission peaks interior to
the bright shell where [O III] and H-beta are strongest. Based on the observed
correlations, we propose that the strong recombination line emission in
planetary nebulae is a result of enhanced dielectronic recombination in hot gas
in the nebular interior, perhaps driven by a hot stellar wind.
