Sodium niobate (NaNbO3) is a perovskite oxide and a key component of emerging lead-free antiferroelectric capacitors for high-energy-density applications. However, its performance can be hindered by irreversible phase transitions and leakage currents associated with low electrical resistivity. Defect and doping engineering offers a potential way to overcome these problems, but its use requires a detailed understanding of electronic, ionic, and polaron charge-compensation mechanisms, where the role of polarons remains largely unexplored. Here, we investigate the stability of small hole and electron polarons in rhombohedral NaNbO3, which is a structurally well-defined model system that avoids lattice-dynamical instabilities. Trapping energies are calculated using density-functional theory corrected by a Hubbard U, using the enforced-piecewise-linearity approach including finite-size scaling. For the small hole-polaron centered on O-2p orbital, we find a trapping energy of -0.65 (eV) and an adiabatic migration barrier of 0.32 (eV) determined by nudged-elastic-band calculations. In contrast, we show that excess electrons do not self-trap on Nb-4d orbitals, reflecting weak electron-phonon coupling in the conduction band manifold. These results identify oxygen as an intrinsic hole trap in NaNbO3 and highlight the importance of including hole polarons in defect models of NaNbO3-based electroceramics.