A controlled pot culture experiment was conducted to determine the influence of different photoperiods on the phenological progression, spike differentiation and development of oat varieties differing in maturity. Three varieties (Bayou3, representing a late maturity variety, Baiyan8 representing an early maturing variety and Baiyan2 representing an intermediate maturating variety) were treated with 8, 12, and 16 h photoperiods, in a factorial arrangement with three replications. Phenological traits and apical development were monitored at weekly intervals. At the jointing, booting, heading and grain-filling stages, superoxide dismutase (SOD), peroxidase (POD), malondialdehyde (MDA) and relative cell membrane conductivity (REC) were determined after destructive leaf sampling. Our results showed that the late maturing Bayou3 was sensitive to photoperiod and was unable to complete spike differentiation and reach heading under 8 h photoperiod. Variety Baiyan8, on the other hand was not sensitive to changes in photoperiod. All varieties took longer to reach specific phenological stages and spike differentiation with the 8 h photoperiod. The SOD and POD activities were greater for Baiyan8 than for Bayou3 and Baiyan2. In contrast, Baiyan8 had lower MDA content and lower REC than the other two varieties. MDA content and REC were correlated with photoperiod. Our data indicate that the earliness per se of oat was related with photoperiod insensitivity, and there was also a relationship between foliar antioxidant enzyme activity and the response of varieties to photoperiod.