OP-TEE was initially developed by ST-Ericsson (and later on by STMicroelectronics), but this was before OP-TEE got the name “OP-TEE” and was turned into an open source project. Back then it was a closed source and a proprietary TEE project. In 2013, ST-Ericsson obtained GlobalPlatform’s compliance qualification with this implementation, proving that the APIs were behaving as expected according to the GlobalPlatform specifications.
Later on the same year (2013) Linaro was about to form Security Working Group (SWG) and one of the initial key tasks for SWG was to work on an open source TEE project. After talking to various TEE vendors Linaro ended up working with STMicroelectronics TEE project. But before being able to open source it there was a need to replace some proprietary components with open source components. For a couple of months Linaro/SWG together with engineers from STMicroelectronics re-wrote major parts (crypto library, secure monitor, build system etc), cleaned up the project by enforcing Coding standards, running checkpatch etc.
June 12 2014 was the day when OP-TEE was “born” as an open source project. At that day the OP-TEE team pushed the first commit to GitHub. A bit after this Linaro also made a press release about this. That press release contains a bit more information. At the first year as an open source project it was owned by STMicroelectronics but maintained by Linaro and STMicroelectronics. In 2015 there was an ownership transfer of OP-TEE from STMicroelectronics to Linaro and since then it has been Linaro who is the primary owner and maintainer of the project. But for the maintenance part, it has become a shared responsibility between Linaro, Linaro members and other companies who are using OP-TEE.