The equilibrium structure and potential energy surface of dilithium monoxide, Li2O, have been determined from large-scale ab initio calculations using the coupled-cluster method, CCSD(T), with basis sets of double- through quintuple-zeta quality. The effects of core-electron correlation on the calculated molecular parameters were investigated. The vibrational-rotational energy levels of the (LiOLi)-Li-7-Li-7 and (LiOLi)-Li-6-Li-7 isotopic species were calculated by a variational approach. A comparison with results of recent experimental high-resolution studies is presented.