I assume you mean you’re running Office 365.
You’ll be pleased to know that you can secure your notes in OneNote with a password. Microsoft use AES-128 to encrypt your notes which is more than sufficient however you must manually activate password protection.
If you’ve not changed the default save-to location then it’ll be OneDrive as this ensures all your devices synchronise whether it be Windows 10, Windows Mobile, Android, iOS or MacOS.
Regardless of whether you activate encryption on your notes you’ll still be able access them on any other device. If you’re using iOS you can use TouchID in conjunction with OneNote to avoid having to enter your password on your iPhone/iPad. If you’re using Android or Windows Mobile then you’ll be required to enter your password in order to decrypt your OneNote secured notes.