I just did a quick web search, and it turns out that I am not the only one that LG has told this too:

[Link: televisions.reviewed.com]

"We've discussed this issue with LG's engineers in the past, and it seems that the TVs go through some kind of maintenance cycle periodically to make sure the OLED's cells are resetting properly. We're not sure exactly what triggers this, however, since leaving the TV off for an entire weekend didn't do the trick the first time. We've put a call into LG's people to find out more, but it appears the best way to fix image retention is to simply use your TV normally, turning it off now and again."