Magnetic, relaxation and electronic dynamic parameters of paramagnetic centers stabilized in an initial and fullerene-modified poly(3-octylthiophene) were studied at the 3-cm (9.7 GHz) waveband EPR in wide (77 K - 320 K) temperature region. Both the systems demonstrate sum spectrum of localized and mobile polarons, which contribution depends on polymer treatment. Both spin susceptibility and spin-spin relaxation rate was shown to govern by dipole-dipole interaction of the mobile polarons. The spin-lattice and spin-spin relaxation times were determined separately by using the steady-state saturation method. It was shown that the treatment of the P3OT sample with PCBM molecules leads to the decrease in activation energy of spin susceptibility, dipole-dipole interaction, and interchain diffusion. This is accompanied with the acceleration of spin relaxation and increase in the system dimensionality.