In order to improve the image quality of a transparent OLED display, the preferred tone-curve is required to effectively reduce the influence of transmitted background. To develop such gamma correction, we compared two types of tone-curves; 1) the simple gamma correction which had one gamma value in the entire gray range and 2) the 2-gamma correction which had two different gamma values. In the experiment1, we checked the effect of two types of tone-curves on distinguishability between black and low-gray levels. Both types of tone curve were more distinguishable than 2.2 reference gamma. Although simple gamma correction had the highest distinguishability, the difference between two types decreased as the correlated color temperature (CCT) of surround lighting became lower. In the experiment2, the preferred type of tone-curve was investigated in a real transparent OLED display under various ambient surrounds. We analyzed the ratio that participants selected the preferred one between two types of tonecurves. Although simple gamma correction was chosen more, the difference decreased as the CCT of surround lightings became lower. Especially, this trend appeared clearly when the images for Public Information Display were presented. These results showed that natural images with simple gamma correction and images for PID with the 2-gamma correction were preferred.