Due to the sophisticated coatings, cleaning OLPFs can be somewhat tricky. First of all assess in camera whether marks visible to the eye actually show up in your image. Don’t clean unless essential. The design of the camera is such that is much more forgiving than you would expect. Bear in mind that with any diffusion filter you will potentially see the filter particles if you shoot at a very deep f-stop, and it is often only at that deep stop that dirt on the filter also becomes visible.
A bulb type blower for dust removal should be the first tool. If that does not work, remove the OLPF from the camera and use canned air to blow the glass clean. Note: keep the can motionless and upright, and move the filter around, not vice-versa.
For grease and smears, any kind of lens cleaning fluid is acceptable. Our factory uses pure alcohol, but again, do this with the filter removed from the camera. Use multiple soft lens tissues, discarding after each wipe.