Please cite R.oo/R.methodsS3 as:

  Bengtsson, H. The R.oo package - Object-Oriented Programming with
  References Using Standard R Code, Proceedings of the 3rd
  International Workshop on Distributed Statistical Computing (DSC
  2003), ISSN 1609-395X, Hornik, K.; Leisch, F. & Zeileis, A. (ed.),
  2003

A BibTeX entry for LaTeX users is

  @InProceedings{,
    author = {Henrik Bengtsson},
    title = {The {R.oo} package - Object-Oriented Programming with References Using Standard {R} Code},
    booktitle = {Proceedings of the 3rd International Workshop on Distributed Statistical Computing (DSC 2003)},
    year = {2003},
    editor = {Kurt Hornik and Friedrich Leisch and Achim Zeileis},
    address = {Vienna, Austria},
    month = {March},
    issn = {1609-395X},
    url = {https://www.r-project.org/conferences/DSC-2003/Proceedings/Bengtsson.pdf},
    howpublished = {https://www.r-project.org/conferences/DSC-2003/Proceedings/},
  }