The Deltora Book of Monsters
Josef was hunted by two Ols disguised as fruit sellers after he spoke to a Dread Gnome about Gellick. They tracked him down to the sewers where he lived with Ranesh, but were killed by Steven and Nevets, who had been following the Ols in turn.
Maze of the Beast
Lief, Barda and Jasmine encountered two Ols near the River Tor. The Ols had been following the companions and eventually took the shape of two girls named Marie and Ida. Initially the companions fell for their trap but Dain was there to rescue them.
While travelling with Dain, Lief and Barda saw two water birds and they tried to feed them. The birds were not interested in food and eventually spread their wings. It was then that Lief noticed the mark of the Shadow Lord and knew that they were Ols.
On the River Queen, Lief, Barda and Dain met several passengers. One of the passengers was a lady in a Pink dress. She was on the boat with her partner. When the pirates attacked the boat Jasmine lost her disguise and the lady in pink recognised her. She changed to her true shape - the shape of an Ol - and proceeded to attack Jasmine. The Ol was eventually killed by the captain of the River Queen before it could kill Jasmine.
Barda had been knocked off the River Queen during the pirate attack. Jasmine and Lief thought Barda had drowned and despaired because they had lost almost everything including the belt. Then suddenly Barda appeared and the companions continued to search for the pirates who had the belt although the pirates were not aware of this. Eventually the companions wandered into a cave and found a note. The note had been signed by doom and was written for the pirates. Lief and Jasmine realised that Doom had betrayed them and that there was little hope of getting the belt back and obtaining the gem from the Maze of the Beast. Barda urged them to give up and go home. At that moment the real Barda appeared and killed the other Barda who was in fact an Ol all along since he had joined them again after the pirate attack.
Initially this Ol had the shape of the man who accompanied the pink lady on the River Queen. They were both Ols and travelled together, just like Ols normally do (grade 1 and grade 2 Ols). The Ol that had the shape of the man on the River Queen and later the shape of Barda, had held the shape of a white water bird that Lief as well. This bird was similar to the water birds that Lief, Barda and Dain encountered, the water birds that were also Ols. It is unclear whether the Ols on the river queen are the same as the water birds that the companions encountered earlier.
Valley of the Lost
Lief, Barda, and Jasmine encountered a group of Ols disguised as goats guarding the King's Bridge. The companions watched them go through the Tremour before continuing along the road.
Near the city of Tora the companions encountered Doom and Neridah and suspected that they were Ols for a moment. This is however proven false when they both walked through the magic tunnel of the city. If they were Ols the magic of Tora would have killed them.
Return to Del
Lief used the water of the Dreaming Spring to spy on Fallow. He realised that Fallow must be an Ol because of the words Fallow had said to his father during an earlier dream.
During the journey to Del, Lief, Barda and Jasmine hid in Steven's wagon. Steven informed the companions that they passed by many Ols. All the Ols were however not interested in Steven and his caravan.
The companions and the Resistance embarked on a rescue mission to save Dain after he was supposedly taken by Ichabod. They all believed that Dain was the heir to the throne and that only he could wield the belt. Lief wore Dain's dagger during the journey to Del but in the palace he realised that he had lost it somewhere. When Lief cut the ropes that bound Dain, he saw that a trap had been prepared for Steven and Nevets. Several rats changed to their true form - the shape of an Ol- and attacked Steven. It was then that he also saw the dagger on Dain's belt and realised that Dain had been a traitor all along. A grade 3 Ol created to deceive everyone and serve as a false heir. Dain lashed out at Lief and knocked the belt into a nearby fire. While Lief talked to Dain about his role, Doom tried to attack him from behind. However, Doom was unable to defeat Dain because of his speed and shapeshifting abilities. Doom's attack bought Lief enough time to recover the belt from the fire and clasp it around Dain's waist, killing him.
After Dain had been killed by Lief, the Ols at the palace were stunned for a moment. The companions including Doom used that moment to flee the palace and plan their next move. They went to the forge but eventually the guards came and they captured Barda, Jasmine and Doom. Lief hid in his room until the guards left. In the chaos of Lief's final effort to reveal the heir of Deltora, the grade 3 Ol Fallow was killed by a red cone.
When finally the Shadow Lord's power was undone in Deltora, all the remaining Ols in Deltora were dying. 
After the Shadow Lord was banished back to the Shadowlands, it decided to replace the Ols with the Conversion Project, since Ols had proven too prone to human weakness. The last Grade 3 Ol created, was 3-19. Lief, Barda and Jasmine spotted 3-19 in the Shadow Factory. In the Shadow Arena Lief saw several Ols entertaining themselves by changing shape. 3-19 was killed at last when he hesitated to start the conversion project.
An Ol's real form resembles ghastly, flickering white flames, as well as smoke, or a boiling raising mass. They have holes for eyes (which sometimes is described as glowing red) a toothless mouth. They have very long arms with strong hands which they often use to strangle their victims. In this form they have the mark of the Shadow Lord at their core.
Whatever shape an Ol takes the mark can always be found somewhere on its body, though it can be in different sizes. In Marie and Ida's case it was a little mark on their chin. Ols always try to hide the mark with clothes or along with normal marks, such as tattoos. To defeat an Ol, one must pierce their heart, placed on the right side as opposed to the left. When they die, they melt into a mass of white and parts of the things and peoples they have taken form of is being revealed in the white mess.
Can take the shape of living people they have seen, but are very cold, cannot eat or drink, have the mark of the Shadow Lord easy to see, and must take a break from the shape every three days. When they take a break they falter and shimmer. This is called "the Tremor". They always travel in pairs.
Cunning and difficult to recognise. They can successfully pretend to eat and drink, make their skin feel warm, hide the mark very well, and can travel on their own. They do however, still suffer from the Tremor every three days. 
Grade 3 Ols can take the shape of non-living things as well as living creatures and are able hold the disguise indefinitely. They are extremely difficult to recognise, as they are almost always warm, eat and drink, do not change shape often, and hide the mark almost at an unrecognisable state. However, this perfection comes at the cost of vulnerability, as a grade 3 Ol in the shape of a human can still die as a normal human would.
The Tremor is the process that Ols go through when they hold a shape for three days. The Ol's control over their shape weakens, and they momentarily return to their true form. The process is very quick, but usually all the time needed to figure out what the Ol is. Grade 1 and 2 Ols experience the Tremor, but grade 3 Ols are able to hold their shapes indefinitely.
The role of Ols in the anime is generally the same as in the books. However, Dain and Fallow do have extended screen time and an elaborated plot in comparison with the books.
Special low level ols with the ability to merge. The final form is stronger and larger depending on the number of Ols merged. Because of its height, its weakness point is harder to reach. Also, they can separate if more numbers are to the Ols' advantage.
Ols have no need or desire to eat, regardless of any form or transformation they are in. They can, however, pretend to eat.
Grade 1 or Grade 2
- Marie and Ida
- Sandra (the lady in pink) on the River Queen
- James (the dancing partner; fake Barda) on the River Queen
- A herd of goats stationed by the King's Bridge.
- The two fruit-sellers (they only appear in The Deltora Book of Monsters, and they are seen and discovered by Josef)
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Rodda, Emily. The Deltora Book of Monsters. Scholastic Australia. 2001.
- ↑ 2.0 2.1 2.2 2.3 Rodda, Emily. The Maze of the Beast. Scholastic Australia. 2001.
- ↑ Rodda, Emily. The Valley of the Lost. Scholastic Australia. 2002.
- ↑ 4.0 4.1 Rodda, Emily. Return to Del. Scholastic Australia. 2002.
- ↑ 5.0 5.1 Rodda, Emily. The Shadowlands. Scholastic Australia. 2002.