We report measurements of the absolute excitation frequencies of Ho165 4f116sns and 4f116snd odd-parity Rydberg series. The states are detected through depletion of a magneto-optical trap via a two-photon excitation scheme. Measurements of 162 Rydberg levels in the range n=40–101 yield quantum defects well described by the Rydberg-Ritz formula. We observe a strong perturbation in the ns series around n=51 due to an unidentified interloper at 48515.47(4) cm−1. From the series convergence, we determine the first ionization potential EIP=48565.910(3) cm−1, which is three orders of magnitude more accurate than previous work. This work is an important step towards using Ho atoms for collective encoding of a quantum register.