In this paper novel results are reported on engagement with ICT standards and their implementations in open source software (OSS). Specifically, findings draw from observations and analyses related to the ITU-T H.265 standard and five OSS projects (x265, libde265, SVT-HEVC, Kvazaar, and Turing codec) implementing the standard. The study reports on a characterisation of organisations that engage with standard development for the H.265 standard and organisations with declarations for H.265 in the ITU-T patent database in relation to standard development for H.265, and how OSS projects implementing H.265 are related to standard development for H.265.