The church was built in the middle of the 13th century and was dedicated to James the Greater. Originally, the church had three bays (wall sections). However, due to the construction of the church tower in the 14th century, half a bay was lost, leaving two and a half bays. The south facade features decorative brickwork in the style of Romanesque Gothic. What is unusual is that the saddle roof tower, probably placed in the 16th century, is perpendicular to other saddle roof towers in the region. The wide side of the tower is against the church. A bell hangs in the tower, dating from 1466 and made by Henrik Kokenbacker for the church in Feerwerd, located northwest of Oostum. Two small buttresses were built against the west facade in 1821.
Oostum was ecclesiastically combined with Garnwerd for centuries. Because most churchgoers went to Garnwerd and most services were held there, maintenance was increasingly seen as an expensive expense by the church council at the beginning of the 19th century. In 1841, the church council wanted to demolish the church, but the people of Oostum successfully resisted this: the municipality of Ezinge requested the Provincial Board of Supervision of Church Property to oblige the church council to properly maintain the church. However, little maintenance was carried out and in 1857 the church council tried to obtain permission from the Provincial Executive to demolish the church. The people of Oostum again resisted. The Board of Supervision then discovered that the obligatory necessary maintenance had not been carried out by the church the year before (although restoration work had been carried out on the church in Garnwerd), while this had been done according to the annual report and accused the church council of misuse of property. It demanded that the necessary maintenance be carried out after all. A structural inspection also showed that the building was not in as bad a condition as the church council had previously wanted us to believe. A member of the church council resigned, the building was restored and further plans for the demolition of the church were not made.
The roof covering is striking. It consists partly of semi-circular tiles (monks and nuns) that alternately lie with the convex and concave sides facing upwards.
The pulpit with a mannerist baptismal fence dates from 1663. The pew, with the family crest of Lewe van Aduard, was made in the first half of the 18th century.
The church is idyllically situated on the mound of Oostum. The church dates largely from the 13th century and has a peculiar tower. Fragments of a wall painting have been preserved inside.