The OCaml Users and Developers Workshop brings together industrial users of OCaml with academics and hackers who are working on extending the language, type system, and tools. Previous editions have been co-located with ICFP 2012 in Copenhagen, ICFP 2013 in Boston, ICFP 2014 in Gothenburg, ICFP 2015 in Vancouver, ICFP 2016 in Nara, ICFP 2017 in Oxford, ICFP 2018 in St Louis, ICFP 2019 in Berlin, and was virtual for ICFP 2020, following the OCaml Meetings in Paris in 2010 and 2011. OCaml 2021 will be a virtual workshop, co-located with ICFP 2021. Please contact the PC Chair (Frédéric Bour) for any questions and see the homepage at https://icfp21.sigplan.org/home/ocaml-2021