The absorption spectrum of the O-18(3) isotopologue of ozone was recorded by cavity-ring-down spectroscopy between 7920 and 7985 cm(-1) with a routine noise equivalent absorption, alpha(min), on the order of a few 10(-11) cm(-1). This permitted to detect and analyze an extremely weak vibrational band assigned as 7 nu(1) + nu(3) following predictions from the ab initio potential energy surface reported in J Chem Phys 2013;139:134307. This is the most excited rovibrational band of the O-18(3) isotopologue detected so far. In total, 259 ro-vibrational transitions were measured and modeled using an effective Hamiltonian model. The vibrational dipole transition moment parameter of the band deduced from the analysis and the fit to 177 experimental line intensities was evaluated as 2.6 x 10(-6) Debye. The absorption coefficient of the recorded pure O-18(3) spectrum was found significantly smaller compared to that of the O-16(3) in the same range. The set of 171 upper state energies experimentally determined cover the range from 92.6% (at J = 1) to 97% (at J = 26) of the first dissociation threshold D-0. The comparison with ab initio predictions for the band origin and nu-dependent rotational constants is discussed. An absorption line list containing 477 transitions is provided as Supplementary material for the 7 nu(1) + nu(3) band. (C) 2021 Elsevier Ltd. All rights reserved.