Any publication referencing OR-Tools should cite it as follows, changing the version number as appropriate:
OR-Tools 7.2. Laurent Perron and Vincent Furnon. https://developers.google.com/optimization/.
BibTeX:
@software{ortools,
title = {OR-Tools},
version = {7.2},
author = {Laurent Perron and Vincent Furnon},
organization = {Google},
url = {https://developers.google.com/optimization/},
date = {2019-7-19}
}