There is only one file type that uses the .OPLC extension:
Nokia Operator Logo File
Image file used by some Nokia mobile phones; stores the manufacturer's logo in a .PNG format; used for displaying the company logo on the Nokia startup screen; can be opened with any image viewer that supports the PNG format.
To upload a custom operator logo to your phone, first upload a PNG image whose file extension has been renamed from ".png" to ".oplc." Then, on the phone, replace the existing Logo.oplc (or whatever logo filename prefix the phone uses) with your new OPLC file.
NOTE: To make OPLC file viewing easier on desktop computers, simply rename the ".oplc" file extension to ".png" and then double-click it to open the file in the default image viewer.
Updated: November 1, 2012