0You should use ONE dictionary for this, and use whatever it says. If you have Oxford, then use check-up. If you have M-W, then use checkup. 02br 02br 00Make whichever dictionary you choose your sole reference for issues of hyphenation - unless, of course, your employer/teacher uses a different one and tells you that THAT one is the one to use.02br 02br 00
In the Collins English Dictionary the word appears as 'Checkup'. Oxford Dictionary writes it with a hyphen: 'Check-up'. It seems as it may be a matter of personal preference, then.