Determining the OID of a certain type

Hi *,
I wanted to ask, if there is a method to determine the oid of a certain type using the C backend interface? I need to get access to a specific type for an IAM and therefore i want to construct an IndexTuple using the specified methods. However, I need the OID of the specific type for it. I considered writing some auxiliary function that returns the OID of the type using plpgsql. This works fine. However, I thought about something that I can use directly in C (server side) to get the oid, or do I have to set up a query to the database? Querying is currently the way I go here. However, it does not feel so right, to return the OID for the type I want to use, using a query. I hope, that there is some predefined function that I haven't found yet to determine the OID of some type (or relation etc).

Best regards,
Carsten Kropf