Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
offertōrĭum (obf-). ii, n. [offero], a place to which offerings were brought, an offertory (eccl. Lat.): offertorium tali ex causā sumpsit vocabulum. Fertum enim dicitur oblatio, quae altari offertur, et sacrificatur a pontificibus, a quo offertorium nomina tur, quasi propter fertum, Isid. Orig. 6, 19.