In order to determine an accurate equilibrium structure for 1,3,4-oxadiazole, microwave transitions and ground-state rotational constants are reported for the parent species and for the O-18 isotopologue measured in natural abundance. These rotational constants along with those of the C-13, N-15, and D-1 species were used together with vibration-rotation constants calculated from a cubic force field calculated at the B3LYP/6-311+G(3df,2pd) level of theory to derive a semi-experimental equilibrium structure. However, the results of this fit were not satisfactory; therefore, the structure was later significantly improved by the mixed estimation method. In this method, internal coordinates from good-quality quantum chemical calculations (with appropriate uncertainties) are fitted simultaneously with moments of inertia of the full set of isotopologues. The accuracy of this structure has been confirmed by using an extrapolation technique. All elements of the N-14 nuclear quadrupole coupling tensor have been determined.