O Is For Onager...

Well, I've searched high and low for armor components beginning with the letter 'O' and have come up with zero.

There is a medieval weapon, however, and that is the onager.

Onager

The onager was a Roman siege engine that is a type of ballista that uses a torsional force, generally from twisted rope, to store energy for the shot. The onager consisted of a large frame placed on the ground to whose front end a vertical frame of solid timber was rigidly fixed. A vertical spoke that passed through a rope bundle fastened to the frame had a sling attached which contained a projectile.
To fire it, the spoke or arm was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. As the sling swung outwards, one end would release, as with a staff-sling, and the projectile would be hurled forward. The arm would then be caught by a padded beam or bed, when it could be winched back again.

That will have to do for this letter. See you all tomorrow!