There have been some studies to examine how people perceived images presented on a transparent display, but there is little research to conduct experiments using a real transparent OLED display. In order to increase visibility of images presented in a transparent OLED display, we find the preferred display gamma value. Before conducting main experiment, we examined the pre-test that showed the change of visibility as surround luminance increased. We confirmed that visibility of low gray levels on the image degraded when the surround condition was bright. Next, we investigated the preferred display gamma value of a transparent OLED display under various surround conditions. The result showed that the preferred gamma value decreased as surround luminance increased. Finally, we explored the cause of lowering of the preferred gamma value by measuring distinguishability of various gammas. This result implied that the lower gamma might be preferred because distinguishability of the low gray levels on the images increases as the surround luminance increases.