Light propagation in photonic band gaps in two-dimensional organic octagonal quasiperiodic photonic crystal slabs is investigated by finite-difference time-domain method. The transmission property and light localization in the polystyrene air-rod slab and air polystyrene-rod slab are compared in detail. The results show that even in extremely low-index dielectric contrast of rods, the photonic band gaps and eigenmodes are observed in the visible spectrum. Besides, the central position of bandgap is red-shifted with the increase of slab thickness. When defects are introduced into two quasiperiodic structures, the occurrence position of defect modes and property of red-shifting in wavelength of modes are different with the increase of size of defect nanocavities. The difference in property originates from the competition consequence of two physical mechanisms which are the energy levels of defects in photonic crystals and the resonance of modes in the defect cavity. The results may give theoretical support for fabricating luminescent devices based on organic quasicrystals.