In this article we use phase resolved optical emission spectroscopy to study emission pattern in plasma sheath of dual frequency capacitively coupled plasma in Ar and Ar-O2 discharge. Two emission patterns are found in sheath region of radio frequency coupled powered electrode. The first pattern is related to electron impact excitation because of the sheath expansion. The second pattern is caused by electron impact excitation of secondary electrons. Two emission patterns are also highly modulated with the low frequency cycle. Under the condition of argon discharge, the emission intensities of the two excitation processes are very similar. The emission structure by secondary electrons becomes weak with the increase of O2 content in the gas mixture. In addition, we also use phase resolved optical emission spectroscopy to study low frequency cycle averaged axial emission profile of excited atomic argon at 751 nm in Ar-O2 mixture gas. Distance from the powered electrode (about 3.8 mm) is defined as the boundary sheath of dual frequency capacitively coupled plasma.