The difference is in the Warranty, in certain cases it is not applicable for OEM products; else even if it is applicable it is for a reduced time-period vis-á-vis the boxed product.
The boxed products get full warranty as specified on the packaging.
As long as you buy an OEM dvd writer from a reputed shop or a shop in which you trust, they will honor one year warranty. I haven't seen anyone complaining about OEM version.
In my case, I was getting LG OEM writer at 950/- and Asus boxed writer at 950/- too,so no brainer, I went with asus one.