We study the spectroscopic evolution of superluminous supernovae (SLSNe) later than 100 days after maximum light. We present new data for Gaia16apd and SN 2017egm, and analyse these with a larger sample comprising 41 spectra of 12 events. The spectra become nebular within 2-4 e-folding times after light curve peak, with the rate of spectroscopic evolution correlated to the light curve timescale. Emission lines are identified with well-known transitions of oxygen, calcium, magnesium, sodium and iron. SLSNe are differentiated from other Type Ic SNe by a prominent O I λ7774 line and higher-ionisation states of oxygen. The iron-dominated region around 5000Å is more similar to broad-lined SNe Ic than to normal SNe Ic. Principal Component Analysis shows that 5 'eigenspectra' capture 70% of the variance, while a clustering analysis shows no clear evidence for multiple SLSN sub-classes. Line velocities are 5000-8000 km s −1 , and show stratification of the ejecta. O I λ7774 likely arises in a dense inner region that also produces calcium emission, while [O I] λ6300 comes from further out until 300-400 days. The luminosities of O I λ7774 and Ca II suggest significant clumping, in agreement with previous studies. Ratios of [Ca II] λ7300/[O I] λ6300 favour progenitors with relatively massive helium cores, likely 6 M , though more modelling is required here. SLSNe with broad light curves show the strongest [O I] λ6300, suggesting larger ejecta masses. We show how the inferred velocity, density and ionisation structure point to a central power source.