Have been looking for but couldn't find any licensing information regarding IBM OneUI.
The one thing I see is a header inside core.css file, which says: "Use, duplication or disclosure restricted by GSA ADP Schedule Contract with IBM Corp."
Is it free to use in internal company's projects?
In the freely available documentation guidelines it says:
"This code enables a common look and feel and a common code base for Lotus® web applications. The goal is to remove arbitrary differences between products thereby improving ease-of-use and making it easier to customize products when deployed in an enterprise."
and in the legal notices it says:
"You may copy, modify and redistribute any Sample HTML contained in this document in any form without payment to IBM, for the purposes of extending functionality and/or interfacing to IBM Lotus products."
This suggests to me that the entire purpose of OneUI is to be used for applications built by companies who have licenses to deploy Notes Applications and that IBM encourages people to do so. More to the point, nowhere in that document does it explicitly say you can't.
So, while not categorical, I'd say it's a safe bet to use it freely on your Notes apps.