Nexus5 has Full-HD on a 5" screen, pushing it to 445dpi while Oneplus has the same resolution on a 5.5" screen resulting in 401dpi.
Especially close-up you will see that your OPO's screen looks less "real" and more computer-screenish. Comparing it to other phones (including my GS3) makes it look very beautiful so it's rather relative.
I do hope that is the cause of your issue and not a real physical layer of dust.
Samsung S1 ↠ Samsung S3 ↠ Oneplus One