Organic spintronics has been an active research field since the demonstration of large magnetoresistance in a thick organic spin valve, in which carrier spins are injected into the organic film sandwiched between two ferromagnetic electrodes. In such spin-injection devices, spin relaxation time and spin diffusion length are key properties that control spin-dependent transport and dictate the device design. Here, we compare spin relaxation and diffusion behaviors in organic solids due to spin-dependent interactions including spin-orbit coupling (SOC), hyperfine interaction (HFI), and exchange. It is found that for SOC-induced spin relaxation, the spin diffusion length is essentially determined by the spin admixture parameter and insensitive to magnetic field and carrier hopping rate, whereas for HFI-induced spin relaxation, it increases rapidly with both the magnetic field and hopping rate. In devices with high-density carriers, where exchange-induced spin motion dominates over carrier hopping, the spin diffusion length is limited by SOC.