OH radical reactivity in the semi-urban atmosphere was measured by a laser flash pump and the probe technique which was developed by our group. The observation OH reactivity during the hole of the year was carried out as well as ancillary measurements of VOCs (Volatile Organic Compounds), NOx, and CO. We defined the oxidant potential, Φ, which tells the number of accumulated peroxy radicals through the radical propagation cycle from a single OH radical. We compared the Φ value derived from the observational results of the OH reactivity measurements with those estimated by the ancillary measurements of VOCs. Under a higher NOx regime, usually higher than 30 ppb, there is no difference between them. However, for cases of cleaner conditions there is a huge difference between the measured and calculated Φ values since the radical propagation cycle will produce large non-linear changes in the production of ozone. We successfully applied the Φ value to the interpretation of the weekend effect of ozone concentration observed on our campus.