Efficient generation of ozone by cold atmospheric plasmas is interesting for sterilisation and decontamination of thermally-sensitive surfaces. This paper presents a study of robust coplanar dielectric barrier discharge (DBD) for generating atmospheric pressure plasma in synthetic air and in oxygen. The atmospheric plasma generated by coplanar DBD in synthetic air showed considerably high ozone production of 2.41g/h (2.25slm, 45W), while the production yield and energy cost were 54g/kWh and 40.9eV/molecule. The use of oxygen instead of synthetic air, at much lower discharge power (2.25slm, 17W), maintained the ozone production of 2.35g/h, whereas the production yield significantly increased to 138g/kWh with a corresponding energy cost of 12.9eV/molecule. The temperature of coplanar DBD ceramics in synthetic air (45W) and oxygen (17W) plasma generation (continuous alternating-current operation) showed temperatures below 70 degrees C and 30 degrees C, respectively. The rotational temperatures obtained from optical emission spectroscopy indicated similar gas temperatures in the thin plasma layer close to the surface of the DBD ceramics. The low temperature of the plasma-ceramics interface evidences the applicability of coplanar DBD for the contact treatment of thermally sensitive surfaces where a high concentration of ozone is required.