To cite osDesign in publications use:

  Sebastien Haneuse, Takumi Saegusa, Thomas Lumley (2011). osDesign: An
  R Package for the Analysis, Evaluation, and Design of Two-Phase and
  Case-Control Studies. Journal of Statistical Software, 43(11), 1-29.
  URL https://www.jstatsoft.org/v43/i11/.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {{osDesign}: An {R} Package for the Analysis, Evaluation, and Design of Two-Phase and Case-Control Studies},
    author = {Sebastien Haneuse and Takumi Saegusa and Thomas Lumley},
    journal = {Journal of Statistical Software},
    year = {2011},
    volume = {43},
    number = {11},
    pages = {1--29},
    url = {https://www.jstatsoft.org/v43/i11/},
  }