OP-TEE is an open source Trusted Execution Environment (TEE) implementing the Arm TrustZone technology. OP-TEE has been ported to many Arm devices and platforms. Originally it was developed as a proprietary TEE solution by ST-Ericsson that later on was moved over to STMicroelectronics.
Back in 2013, Linaro formed the Security Working Group (SWG): one of the initial key tasks for the SWG was to work on an open-source TEE project. After talking to various TEE vendors, Linaro began working with STMicroelectronics to convert their TEE solution from being a proprietary TEE to become an open source TEE. After a few months of refactoring and rewriting major parts of the code so it was compatible with the BSD 2-Clause license it was released to the general public around the summer in 2014.
In 2015, ownership of the OP-TEE project was transferred from STMicroelectronics to Linaro. Between 2015 and 2019, Linaro was the owner of the project and the maintenance was shared together with the members of Linaro. Late 2019, Linaro transferred the OP-TEE project to the TrustedFirmware.org project, who has been the governing body since then. Linaro is still in charge of scheduling releases, acting on security incidents etc. But when it comes to development and features there are many companies contributing to the project.