High-resolution Coster–Kronig (C–K) electron emission spectra in collisions of 32MeV O 3+ ions with a He gas target have been investigated using zero-degree electron spectroscopy. Zero-degree electron spectrum for the O 3+ projectiles exhibits a trident-like structure around the cusp peak, consisting of three clear peaks. The sharp peaks directly on either side of the cusp corresponded to autoionization lines from highly excited O 3+ ions produced in the collision with a He gas target, which are confirmed to be due to the configuration 1s 2 2s2p( 3 P)5s, where the C–K ejection energy is 0.014eV. The C–K transitions in higher energy region are found to originate in 1s 2 2s2p( 3 P)nl–1s 2 2s 2 εl′ (n=5–11). For n=5–7, a clear angular momentum distribution is observed, where the maximum intensity is attributed to the relatively high angular momentum l=2 or 3.