First off, there's a 6-digit keypad. This is key -- pardon the pun -- to much of what makes the OnlyKey different.
That keypad allows OnlyKey to be protected by a PIN code, and for a second account to be set up, along with a self-destruct PIN code.
These PINs add an additional layer of security, preventing the key from being useful to someone who finds it.
In all, you can store up to 24 passwords, up to 24 usernames/URLs, and up to 24 OTP accounts on a single OnlyKey.
Beyond that, OnlyKey supports FIDO U2F and Yubikey OTP 2 factor authentication for an unlimited number of sites.
The OnlyKey is also open source, has upgradable firmware, and can also be backed up (in case you lose the key and need to restore the data onto another).
Your OnlyKey can be set up using either an app (Windows, Mac, and Linux), or you can choose an app-free quick setup.
There's also very in-depth documentation that guides you through all the features.
Each key also comes with a removable black silicone protective sleeve.