HI,
I have very extensive experience with SAML based integrations. In past i completed SAML integrations with various Identity providers like ADFS, Onelogin, Okta, Shibolith, IBM Tivoli etc. I was part of the team building SAML toolkit for connecting any SAML supported IDP to perform Single Sign On. I can help you setup the SAML integration with tablau.
After going into your project description, i guess the issue is that you need to setup a Active Directory account, which will integrated with both Tableau and Onelogin, by that hvaing the user shared across both the platform, then only you can have the users from Tableau database to have access through OneLogin.
Let us connect to discuss more.