Rotational spectra of small- to medium-sized He-N-OCS clusters with N from 2 to 8 were investigated using a pulsed molecular beam Fourier transform microwave spectrometer and a microwave-microwave double resonance spectrometer. Spectra of five to eight isotopomers were measured for each He-N-OCS cluster. Nuclear quadrupole hyperfine structures were observed for S-33 and O-17 containing isotopomers. The isotopic data, together with the microwave-microwave double resonance experiments, were used to establish unambiguous assignments for these clusters. The assignment of N, the number of solvating helium atoms in a particular cluster, was supported by the observed intensity change under different backing pressure and nozzle cooling conditions and in particular by the systematic infrared vibrational band shifts from the concurrent infrared study. This sequence of clusters contains some of the largest clusters that have thus far been investigated with high-resolution spectroscopic techniques. This study provides detailed and important information for the understanding of the quantum solvation process of dopant molecules in helium nanodroplets. (C) 2003 American Institute of Physics.