Thanks for sharing your work. In the OSBo user manual, I have actually documented the 1.8.3 version: http://rayshobby.net/docs/osbo11_manual.pdf
page 9. It’s different from the version you found, but the differences are probably very minor.
I took a quick look at your github repository, and it seems to use the Penman–Monteith equation, which if I understand correctly is an approximate ET algorithm. Is that correct? Would you mind sharing some more details about it? Thanks.