Open up your mission file (mission.sqm) with notepad/editor and you'll see something like the following:
addOns[]=
{
"ah64",
"oh58",
"su25",
"bis_resistance",
"bmp2",
"sebnam_us",
"sebnam_wp",
"f4",
"editorupdate102",
"sebnam_vc",
"sebnam_vha",
"sebnam_obj",
"sebnam_vh",
"sebnam_ls",
"sebnam_s",
"6G30"
};
addOnsAuto[]=
{
"sebnam_us",
"sebnam_vc",
"sebnam_ls",
"editorupdate102",
"sebnam_obj",
"sebnam_vh",
"sebnam_vha",
"6G30",
"bis_resistance"
};
That top section tells OpF which addons are needed to open this mission. So if you're really sure what addons are still in that list, but were deleted in the mission, you can delete the entries manually and save it again.
Sometimes its hard to recognize which addon is connected to which entry. For example, if you have placed some resistance houses or bushes, they all will be connected to the "bis_resistance" entry.
You can screw up your mission by deleting entries, so i suggest to make a COPY of the original file FIRST.
Wish you luck.