If it’s been a couple of months since you last used your oil finish, the cap can be a bear to get off. Drips of finish around the spout can dry like glue. Once you get the cap off, try putting Teflon plumbing tape (available at home centers for $1 a roll) around the spout before closing it. The next time you need more oil finish, the cap will come right off. Thanks to John Brantner for this tip.