The aim of this study is to examine the effects of ozone treatment applied at two different times, followed by the in vitro application of a reductant solution, on the shear bond strength of four different restorative materials to enamel and dentin. A total of 400 caries-free human molar teeth were randomly divided into enamel and dentin groups, and each was divided into five subgroups. The O20 group received 20-s ozone, O20 + R received 20-s ozone+reductant solution, O80 received 80-s ozone, O80 + R received 80-s ozone+reductant solution, and C was the control group. Each subgroup was then divided into four more subgroups based on restorative material (i.e., nanohybrid resin composite, silorane-based microhybrid resin composite, compomer, or conventional capsulated glass ionomer cement). After water storage for seven days, shear bond-strength data (MPa) were measured using a universal testing machine at a crosshead speed of 0.5 mm/min until fracture, and failure types were examined. The effects of ozone and ozone+reductant applications on enamel and dentin were examined via scanning electron microscopy (SEM). Bond strength data were statistically evaluated using a three-way analysis of variance and Tukey's honestly significant difference test (a= 0.05). The bond strength of the control group was like that of the O20 group (p > 0.05), but it was higher than all other groups regardless of the type of restorative material or tooth structure (p < 0.05). Adhesive-type failures were found in most groups. Via SEM examinations, it was observed that ozone applications did not change the enamel morphology, but they did cause narrowing of the dentinal tubule openings in O80 + R.