Summary link by OSTIF, which includes a quick synopsis of both audits, and link to full report on OpenVPN 2.4 by OSTIF:

https://ostif.org/the-openvpn-2-4-0-audit-by-ostif-and-quark...

Matthew Green/Cryptography Engineering audit direct link, which focuses more heavily on the protocol design, as well as a 2.2->2.4 changelog audit:

https://www.privateinternetaccess.com/blog/2017/05/openvpn-2...