Extensive new data on the odd-parity, single-colour, two-photon optogalvanic spectra of argon in the visible region is reported using an excimer-pumped dye laser in conjunction with an argon discharge cell. The experimental technique enables us to record a pure two-photon spectrum in a region which is dominated by one-photon transitions. A large number of two-photon transitions, originating from the two metastable states 3p 5 4s[3/2] 2 and 3p 5 4s [1/2] 0 to the excited states of configurations 3p 5 ns (J = 0, 2) and 3p 5 nd (J = 0-4) of argon, has been recorded and arranged into 10 Rydberg series. The interchannel interactions among the 3p 5 ( 2 P 1/2 )nd and the 3p 5 ( 2 P 3/2 )ns, nd J = 2 channels and the 3p 5 ( 2 P 1/2 )nd and the 3p 5 ( 2 P 3/2 nd J = 3 channels have been studied using the multichannel quantum defect theory (MQDT). In addition, higher members of the two-photon parity forbidden 3p 5 np series have also been detected.