Work on the cathedral as a memorial to St. Olav started in 1070. It was finished some time around 1300, nearly 150 years after being established as the cathedral of the diocese. The cathedral was badly damaged by fires in 1327 and again in 1531. The
nave was destroyed and was not rebuilt until the restoration in early 1900s. In 1708, the church burned down completely except for the stone walls. It was struck by lightning in 1719, and was again ravaged by fire. Major rebuilding and restoration of the cathedral started in 1869, initially led by architect
Heinrich Ernst Schirmer, and nearly completed by
Christian Christie. It was officially completed in 2001. Maintenance of the cathedral is an ongoing process. The oldest parts of the cathedral consist of the octagon with its surrounding
ambulatory. This was the site of the original high altar, with the reliquary casket of
Saint Olav, and choir. Design of the octagon may have been inspired by the Corona of
Canterbury Cathedral, although octagonal shrines have a long history in Christian architecture. Similarly, the choir shows English influence, and appears to have been modeled after the Angel Choir of
Lincoln Cathedral. It is joined to the octagon by a stone screen that fills the entire east side of the choir. The principal arch of this screen is subdivided into three subsidiary arches: the central arch frames a statue of
Christ the Teacher, standing on the top of a central arch of three subsidiary arches below him. The space above the principal arch, corresponding to the vault of the choir, contains a
crucifix by the Norwegian sculptor
Gustav Vigeland, placed between statues of the
Virgin Mary and the
Apostle John. Built into the south side of the ambulatory is a small well. A bucket could be lowered to draw up water drawn from the spring that originated from St. Olav's original burial place. (This was covered over by the construction of later cathedrals). The present cathedral has two principal altars. At the east end of the chancel in the octagon is an altar at the site of the medieval high altar, behind which stood the silver
reliquary casket containing the remains of
St. Olav. This silver-gilt reliquary casket was melted down for coinage by
Christian II and St. Olav's remains buried in an unknown location under the cathedral. The only relic known to have survived is a
femur in a silver-gilt reliquary. Shaped as a forearm, it was given by Queen
Josephine to St. Olav Catholic Cathedral in Oslo. The original reliquary casket was in the form of a church, with dragon heads on its gables. The dragons are similar to those carved on the gables of Norwegian
stave churches. Surviving medieval reliquary caskets in Norway frequently also bear such dragon heads, for instance, that at
Heddal Stave Church. He was the church's and the kingdom's patron saint. The current altar was designed to recall in marble sculpture the essential form of this reliquary casket. It replaces the previous baroque altar, which was transferred to
Vår Frue Church. The second altar is in the crossing, where the transept intersects the nave and the chancel. It bears a large modern silver
crucifix. It was commissioned and paid for by Norwegian American emigrants in the early twentieth century, and the design was inspired by the memory of a similar silver crucifix in the medieval church. The medieval
chapter house may also be used as a chapel for smaller groups of worshipers. All the stained glass in the cathedral dates from its rebuilding in the 19th and 20th centuries. The windows on the north side of the church depict scenes from the
Old Testament against a blue background, while those on the south side of the church depict scenes from the
New Testament against a red background. ==Organs==