Korean news site ETNews reports that Samsung plans to release two main models of its upcoming Galaxy S5 flagship phone - a standard plastic-bodied flat phone and a premium one that will sport a flexible OLED and a metal case. It'll be interested to see whether Samsung simply curved the premium S5 (like the Galaxy Round) or adopt a more radical design (wrap-around display or even a foldable one, although frankly these are only expected in 2015).
ETNews reports that initial production will begin in January 2014, and mass production will begin in February. This was also the case with the Galaxy S4 (which was announced on March 15 2013). It's unlikely that Samsung will have enough flexible OLED capacity to supply such a flagship phone so it makes sense they will only adopt it in a premium model. This is exactly what Samsung did with the flat Galaxy Note 3 and the curved Galaxy Round.
It'll be very interesting to see what kind of display will be adopted for the GS5. The GS4 uses a 4.99" Full-HD (1920x1080, 441 PPI) Super AMOLED display behind a Corning Gorilla Glass 3. The display uses Samsung's Diamond Pixel architecture.