Well I'm not a Sharepoint or SF expert but I have done many OAuth token requests.  If the resource you are trying to access is on Sharepoint, then they should be able to provide the support for you.  The basics are, you should have some sort of service account setup for Sharepoint.  The service account will have a client_id and client_secret associated to it.  There's several different oauth grant types, but most common are password flow and client credentials.  With both, you make a token request with your client_id and client secret and get an access token returned.  In the password flow, you need to give your userid and pw as well.  In some cases, the token either never expires, expires in a year or 2, or sometimes less.  If it's less, they usually give you a refresh token you can use to make another request.  Some organizations, like DropBox, do it a little different.  They allow you to generate the oauth token when you create your connected app (service account) right from their developer console just by clicking a button.  You'll have to contact Sharepoint, or Google how their access tokens and service accounts work.  You really need to find the code that makes the token request though, you should just be able to run it again to generate a new token.

Good luck,
Kevin