Octogonal (octagonal)

Although not as numerous as the square variant, there are quite a few octagonal clochers (bell towers) in Burgundy.
The picture shows the only remaining octagonal bell tower of the abbatiale (abbey church) of Cluny.

Clocher de l'abbaye - Cluny

Oculus (oculus)

A small circular opening, often in the top of a coupole (cupola).
The oculus, when installed as a window in a wall, is also called œil-de-bœuf.
One can find three oculi in the façade (facade) of the Saint-Barthélémy in Fleury-la-Montagne.

Saint-Barthélémy - Fleury-la-Montagne

Ogive (rib)

Load bearing element, placed on the intersection of two crossing voûtes (vaults).
A vault with this shape is called a voûte d'ogives or croisée d'ogives (rib vault).
The ribs of the vault are actually supporting the planes of the vault.
In a voûte d'arêtes (groin vault) the planes are self-supporting.
The picture shows the ribs in a chapelle (chapel) in the Saint-Julien in Sennecey-le-Grand.
See also voûte d'ogives and croisée d'ogives.

Saint-Julien - Sennecey-le-Grand

Opus piscatum - entry withdrawn

 This term was used in the past in Wikipedia, but has been more or less withdrawn.
See opus spicatum and arête de poisson (synonym).

Opus sectile (opus sectile)

A technique, whereby pieces of different materials (marble, glass, mother of pearl) are cut and inlaid into walls or floors to make a picture or pattern.
One of many variants of mosaic technique.

Opus sectile - Musée Rolin in Autun

Opus spicatum (ears of wheat bond ©)

The translation in the title is followed by this sign ©; this means that it has been "invented" by me.

It is a stone bond in the shape of a herringbone, and found in carolingien (Carolingian) and préroman (Pre-Romanesque) buildings.
There is some confusion about this term in French Wikipedia. They distinguish "appareil en épi de blé" = stone bond in the shape of ears of wheat (opus spicatum) and "arête de poisson". The difference is that in case of arête de poisson the joint between the herringbones forms a straight line, while in opus spicatum it follows a zigzag line.
It is also used in pavements.
The picture shows this pattern in a (recent) piece of brickwork in the chauffoir (calefactory) of the abbaye (abbey) of Fontenay.
See also Arête de poisson.

L'abbaye de Fontenay

Orant (orans)

A depiction of a praying person, with his or her arms raised in prayer.
The sculpture shows a member of the clergy in prayer, and the sculpture adornes the portail (portal) of the Chartreuse de Champmol in Dijon.

Chartreuse de Champmol - Dijon

Orientation (orientation)

Orientation means that the axis of the church, from the western entrance to the eastern chœur (choir) is facing the astronomical east (towards the orient).
The abside (apse) is the extreme eastern part of the church.

Orientation - internet

Ottonien (Ottonian)

The Ottonians or Liudolfings were kings from a Saxon dynasty which ruled from 919-1024.
The kings of this house became emperors of the Holy Roman Empire.
Ottonian churches are charactarized by having a Westbau (westwork); these churches possessed two chœurs (choirs) and two absides (apses), an eastern and a western one.
The picture (Wikipedia) shows the family tree of the Liudolfings.

Family tree of the Liudolfingen - Wikipedia

Oves (oval decorations)

An ornament showing a number of sculptured, egg shaped patterns.
The picture shows the portail (portal) of the Saint-Pierre in Uchizy.

Saint-Pierre - Uchizy