libpq and referencing data type OIDs

I want to use postgres' datatype OID definitions and not use the literal 
Oid int values (I'm feeding them into PQexecParams()).  But, it looks 
like these values are defined in a header not included by libpq-fe.h.  
When I try importing catalog/pg_type.h, I get a slew of missing data 
type errors.  So, what's the appropriate way to reference OID data types?

-- 
Marvin Keith Bellamy
Software Engineer
Innovision Corporation
913.438.3200