Carbon-linked 5-hydroxymethylfurfural (HMF) oligomers contain at least one first HMF unit and one second HMF unit. The first and second HMF units are linked by a carbon-carbon bond with involvement of an aromatically bound carbon atom at position 3 or 4 of the furan ring of the first HMF unit. The HMF oligomers may be used as reactive carbonyl compounds in the manufacture of thermally curable resins on the basis of phenolic compounds and/or aminoplastic forming agents as well as in the manufacture of wood composite products.