They are inherently self-centered, seeking only their own promotion, achieved by pleasing the Shadow Lord. At the core of every Ol is the Shadow Lord's mark, and in whatever shape the Ol takes, that mark is always somewhere on the body.
Most Ols attempt to cover the mark with clothing or camouflage it with tattoos. Grade 1 Ols and Grade 2 Ols can be killed by stabbing them through the heart, which is located on the right-hand side; given the evidence so far, Grade 3 Ols might not be able to be killed in this manner.
Josef wrote in the Book of Deltora Monsters that he was almost assassinated by two Ols. One day when he was hiding in the streets of Del, he met a Dread Gnome that tried to warn Del's people of Gellick and Ols and other evil creatures. Josef paid little heed until the next morning when he found the gnome dead. The killers were a pair of Ols that hung out near the inn where Josef was staying and they would have assassinated him as well if Steven had not been in the area and allowed Nevets to kill them. Ols come in three types:
- Grade 1 Ols:
- Can mimic any living creature - from the description, size does not matter, but all the Ols shown have been humanoid, or animals at least as large as a woodmouse.
- When they mimic, they leave a brand exposed on their body.
- Are very cold to the touch, and can cause freeze burns during attacks.
- Can not hold their shape for very long.
- Cannot eat or drink.
- Must travel in pairs, although the pairs do not need to look alike and may even be different species.
- Grade 2 Ols:
- Can hold their shape for 3 days, after this there is a break in concentration, a spasm known as "The Tremor", which is an unmistakable sign of an Ol.
- When they mimic, their brand is hidden.
- capable of mimicking the acts of eating and drinking.
- able to produce body heat.
- They are also able to travel alone.
- Grade 3 Ols:
- Can shapeshift into even non-living things.
- Their brand cannot be seen.
- Can hold their shape indefinitely.
- Are indistinguishable from the real person, except by personality (for a convincing performance, the Ol would have to have time to study the one being mimicked).
- Can travel alone.
- Can die like the beings they imitate, for example when Prandine was killed by being pushed out a window by Sharn.
- Marie and Ida
- Sandra (the lady in pink and James' wife) on the River Queen.
- James (Sandra's husband who is the dancing partner; fake Barda) on the River Queen.
- The two fruit-sellers (only appearing in The Deltora Book of Monsters, and seen and discovered by Josef)
- The Seer
- The Foursome (Barba, Jasmine, Filli, Kree)