This work investigates the transitions between ohmic mode and Penning-Gamma mode in a capacitively coupled radio frequency micro atmospheric pressure plasma jets operated in He/O2 mixtures by comparing phase-resolved optical emission spectroscopy measurements of helium excitation with numerical simulations. The simulations employ a hybrid model that treats electrons kinetically via particle-in-cell/Monte Carlo collision simulation, while ions and neutrals are modeled based on fluid dynamics. These results reveal that an increasing electronegativity causes inhomogeneities in the bulk electric field, consequently modulating the electron impact excitation dynamics. A good agreement was found between experiments and simulations.