We report the first O-17 quadrupole-central-transition (QCT) NMR spectroscopic study of protein-ligand complexes in solution. This work shows that it is possible to obtain high resolution O-17 NMR spectra for O-17-labeled ligands bound to proteins. At high magnetic fields such as 21.14 T, this O-17 QCT NMR approach should be applicable to studies of all oxygen-containing functional groups in large proteins (>30 kDa).