Looking at the data sheet, by default it's in 4/8 bit mode so should work the same way as a normal 2 x 16 LCD display. The only suggestion to prove if the OLED is part of a faulty batch would be to run the code above on a different PIC (18F4580 or the good old 16F877A) - If that works then the problem would appear to be with the original pic, if the OLED still gives odd pixels then its the display and I would send them back for a refund and order from a different supplier