OpenAthens Cloud SP is middleware that allows you to use the widespread and simple OpenID Connect software to participate in SAML federations around the world and easily allow institutional subscription to your content.
OpenID Connect (or OIDC) was introduced in 2014 by the OpenID Foundation to deliver a more API-friendly way of performing many of the same tasks as OpenID 2.0. This API-friendly approach is what makes it work well within native and mobile applications as well as OpenAthens. It is, basically, an authentication layer on top of the OAuth 2.0 authorisation framework.
There are versions available for all major platforms and if you are already equipped for social logins such as Google, Facebook or Twitter you may already be using it.
At the basic level, OpenID only does authentication - i.e. is the user who they say they are; OAuth is for finding out information about the user such as identifiers and affiliations. OpenID Connect does both of those, which is what you need when you are dealing with institutional customers.
OpenAthens (originally just Athens) was introduced in 1995 and was one of the first federated access management solutions in the world. It has evolved over time to embrace new technologies as they emerged such as SAML and now OIDC.