The One Plus X at 1080p 5" has 441ppiBecause MRU said that AMOLED vs LCD, you'd need more resolution on AMOLED to match LCD. I was skeptical about that as I haven't really seen that AMOLED displays would look any worse.
The One Plus One at 5.5" has 401 PPI
So with the 1080p one plus X there are 4,147200 Lighting Elements on a 1080P OLED Screen
Compared to the 6 million on the 1080p LCD.
However because you also have to factor in a smaller screen and higher PPI between the smaller device on the X and the larger LCD variant on the OPO, that difference is not as noticeable had the one plus X had a same 5.5" panel to your One Plus One and then you would notice that text wouldn't look as sharp or crisp because of the pentile sub pixel arrangement despite same size & resolution.