We continue our systematic statistical study of various components in gamma-ray burst ( GRB ) optical lightcurves . We decompose the early onset bump and the late re-brightening bump with empirical fits and analyze their statistical properties . Among the 146 GRBs that have well-sampled optical lightcurves , the onset and re-brightening bumps are observed in 38 and 26 GRBs , respectively . It is found that the typical rising and decaying slopes for both the onset and re-brightening bumps are \sim 1.5 and \sim - 1.15 , respectively . No early onset bumps in the X-ray band are detected to be associated with the optical onset bumps , while an X-ray re-brightening bump is detected for half of the re-brightening optical bumps . The peak luminosity is anti-correlated with the peak time , L _ { p } \propto t _ { p } ^ { -1.81 \pm 0.32 } for the onset bumps and L _ { p } \propto t _ { p } ^ { -0.83 \pm 0.17 } for the re-brightening bumps . Both L _ { p } and the isotropic energy release of the onset bumps are correlated with E _ { \gamma, iso } , whereas no similar correlation is found for the re-brightening bumps . These results suggest that the afterglow onset bumps are likely due to the deceleration of the GRB fireballs . Taking the onset bumps as probes for the properties of the fireballs and their ambient medium , we find that the typical power-law index of the relativistic electrons is 2.5 and the medium density profile behaves as n \propto r ^ { -1 } within the framework of the synchrotron external shock models . With the medium density profile obtained from our analysis , we also confirm the correlation between initial Lorentz factor ( \Gamma _ { 0 } ) and E _ { iso, \gamma } in our previous work . The jet component that produces the re-brightening bump seems to be on-axis and independent of the prompt emission jet component . Its typical kinetic energy budget would be about one order of magnitude larger than the prompt emission component , but with a lower \Gamma _ { 0 } , typically several tens .