We report resolution-enhanced magic-angle sample spinning (MAS) O-17 NMR studies of the paraelectric-antiferroelectric phase transition (at T-C = 373 K) of the model hydrogen-bonded compound squaric acid (H2C4O4). Utilization of single crystals in the MAS measurements yielded a factor of 4 decrease in the O-17 NMR line widths, as compared to powder samples. All four oxygens were clearly shown to be chemically different at T < T-C. This was akin to C-13 spectra, but the O-17 peaks are much more dispersed. The peak assignment was supported by quantum theoretical calculations of the O-17 isotropic chemical shifts using a pentamer model of the crystal structure below T-C. On raising the temperature, the four peaks merged and became a narrow doublet above T-C, in contrast to a singlet for C-13. Also, the mean position of the doublet was not the average of the four low-temperature peaks. This observation suggests that the phase transition involves a distortion of the whole H2C4O4 framework, and not just the order-disorder rearrangement of the H's i.e., future models of the transition should include a displacive component, in addition to an order-disorder part. The observation of the doublet at T > T-C implies that the two O-H...O chains retain their difference in the paraelectric phase as well. This is consistent with the one-dimensional Ising chain model, according to which the SQA lattice should be visualized as a mesh of two distinct and orthogonal one-dimensional chains, in contrast to the more prevalent two-dimensional C4O4 square-lattice model.