In the present work, the spatial distributions of OH radicals and O atoms were studied in argon hollowneedle-plate discharge in ambient air. A 3 cm long plasma torch was generated in the discharge. The optical emission spectrum from 300 to 800 nm was collected. Besides ArⅠ lines and N2 second positive band system, OH emission band around 309 nm, O line at 777.4 nm and weaker H line were found in the optical emission spectrum. Because OH radicals and O atoms play an important role in material surface modification, the relative intensities of the OH radicals band (around 309 nm) and O atom line (777.4 nm) were analyzed. The results show that the number of OH radicals decreases rapidly and the number of O atoms follows a rule of increasing firstly and then decreasing from arc root to arc tip.