There's no definitive answer to that question and any answers you do get (including this one !) will be subjective and based on personal experience.That's the case is it... OLEDs less prone to image retention than plasma?
The problem is that there are so many different factors at play - the type and duration of content watched, picture settings, make and model differences and individual panel variations could all have a potential effect on the risk of 'screen burn' on an OLED. While many of those aren't that different to plasma, one issue is that screen burn on an OLED is essentially 'pixel wear' and considered to be cumulative, so 'mixing your content' doesn't have as much of an effect on reducing the risk of image retention as it did on plasmas.
My take on it is this - with plasmas, you often stood a chance at removing any temporary image retention if you spotted it before it became permanent and took action by changing your content for a while, whereas the OLED 'screen burn' issues you often see talked about on these forums tend to be permanent because the pixels have simply worn out - so by the time it starts to appear on an OLED, it's not possible to remove it or remove/reduce the effect.
I take more care with my Panasonic OLED than I did with my plasma, which had proven to be pretty resistant to any kind of image retention, temporary or otherwise... with my viewing habits and picture settings, at least.