C where are oids defined?

Hi, I'm new and I can't seem to figure out what values are permitted for OID
PQexecPrepared(parrentcon, insertstmt, 2, argz_str, //fixme);
I've looked in the docs but it's simply evading me. In C the values are of type char * and in the database they are of
typevarchar(255).
 

Thanks