In the OpenGL tab, select the Enable option in the Full Scene Antialiasing section.
Number or samples: Enter the number of samples you want to be used for the antialiasing process. The number of samples is basically equivalent to the amount of times a pixel will be enlarged to calculate the antialiasing. This technique is called supersampling. The higher the number of samples, the better the antialiasing quality will be, but the heavier it will be on performance.