First of all, I would like to thank everyone here and on Reddit who researched and provided references for their efforts and contributions.
Here is the summary of the findings as they stand today:
1) The term "object-oriented" was known and used throughout the sixties and seventies in various domains, such as developmental psychology, library and information science, and systems engineering.
2) The term was also used in computer science, at least since 1971, e.g. applied to database systems, operating systems, structured exception handling and structured analysis; see e.g. https://archive.org/details/databasesystems0000cour.
3) The term "object-oriented" was applied to a programming language for the first time in the MIT CSG Memo 137 (April 1976) by Anita K. Jones and Barbara H. Liskov; a similar text was published in the IEEE Transactions on Software Engineering later in the year (see see https://ieeexplore.ieee.org/document/1702384).
This is an amazing result and sheds new light on the history of object-oriented programming terminology.
If you want to have a look at the references and discussions yourself, see the comments here and on Reddit: https://old.reddit.com/r/ProgrammingLanguages/comments/156ng....