SAMLIdentityProvider.ReceiveSLO receives the logout request or logout response from the partner service provider.
In this case with Office 365 initiating SAML logout, SAMLIdentityProvider.ReceiveSLO will receive the logout request.
After logging out the user locally, call SAMLIdentityProvider.SendSLO to send a logout response back to the service provider.
These APIs are part of our SAML v2.0 for ASP.NET product.
There are similar APIs for our SAML v2.0 for ASP.NET Core product.
It sounds like you're using the ASP.NET product.
The ExampleIdentityProvider project demonstrates calling these APIs.