Olm (libolm) is an independent Apache-licensed implementation of the Double Ratchet cryptographic ratchet in C & C++, also including the new Megolm group ratchet as used in Matrix.
The Olm repository now lives at https://matrix.org/git/olm.
We don't host Olm on Github as that would constitute re-exporting its cryptography to/from the US, which technically requires us to file additional paperwork with the US government: see https://www.bis.doc.gov/index.php/policy-guidance/encryption/encryption-faqs#6 for details. Given Olm is currently developed in the UK & France, and https://matrix.org/git is hosted in UK / Holland / Germany, we store the code on this side of the Atlantic :)
However, please do file bugs against https://github.com/matrix-org/olm/issues, as we don't have anywhere else to track them!