I'm on an early 2009 iMac running Yosemite. I have tried all the likely passwords and all are rejected. The specific field is labeled: "Keychain Access wants to use the "1Password" keychain. Please enter the keychain password."
What version of 1P are you running please? (Go to 1P > About 1Password and tell us exactly what it says there.)
Version 5.1 (510027), Mac App Store
@gerrydj: Welcome to my nightmare scenario. I had this same issue about a week ago when Keychain didn't properly update my password on some items. Here's how I was able to resolve it:
Between using iCloud to unlock my Mac previously, changing my iCloud password, and then switching back to using a regular user account password, it was a bit of a mess. The hardest part is to even understand which password OS X might be asking for (especially on the login screen, since it just has an empty password field).
From your description, it also sounds like you may have an old 1Password Keychain (pre-AgileKeychain) stored in the OS X Keychain, so keep in mind that this might be a very old password that it is asking for. I hope this helps!