It is not known exactly when the organ at Oosthuizen was built or assembled in its current configuration. Organ specialist Jan van Biezen has already proposed that it is an amalgamation of several older instruments. A document from the church archives dated 1548 and signed 'Dierick Willemsen coster ende orgl in oijsthuijsen' (... warden and organist ...) shows that an instrument existed at that time. The date of thc church's completion -1521- is often taken as the year in which the organ was built as the case, pipes, and keyboard range clearly display characteristics of the end of the 15th and beginning of the 16th centuries.
During the 2002-3 restoration, the pipework was carefully re-examined and restored with great care. The note markings on the pipes bear the handwriting of organ-makers that suggests the assembly was the work of Pieter Backer, active around 1670. He seems to have used and combined parts of other organs, perhaps those previously in the churches of Monnickendam, Hoorn and Medemblick (where Sweelinck was married). The Octaaf 4 stop for example, had served as diapason pipes before their use in Oosthuizen, and may date from the beginning of the 15th century. Backer also worked in Hoorn, where a 15th-century organ was then still in existence, but at that time - as we know, thanks to coster ende orgl Dierick Willemsen - there was already an organ at Oosthuizen, which Backer could have used.
The disposition is not really typical of the 16th century: the Sexquialter register, for example, was not built in the Low Countries before the 17th century. The keyboar compass, on the other hand, does correspond to the standard expected at around 1500: 38 keys from F-G-A to g"-a". From the point of view of the oganisation of the pipework and the decoration, the prospect - aside from some 19th century additions - certainly still displays all the stylistic characteristics of the Late Gothic period.