Olaf Magnusson was king of Norway from 1103 to 1115. Olav was the son of king Magnus Berrføtt (Magnus the Barefooted). He was king in name only, his older brothers Øystein I Magnusson og Sigurd I Magnusson Jorsalfar ruled in his place. He was 16 or 17 years old when he died, didn't have much of an active role in any decicions or administrative decicions, and for that reason he is not counted in the official list of Norwegian kings, even though he technically was king. But, more on that later!

We know little about Olaf, the sources we have say very little about him. He was not much involved with running the country, so to speak. Norway was divived into three areas of administration during his reign, and the sources tell little of what he did. Olaf and his more famous three brothers ruled over each area. The sources we have are full of information about his two brothers; Øystein built ships, harbours, halls and so on, Sigurd Jorsalfar of course (Jorsalfar meaning traveller to Jerusalem, ie he was a crusader) was abroad leading the Norwegian Crusade and making a name for himself. Taxes were relaxed and they "turned slavery into freedom", as one source put it. Olaf is described as tall, blonde, good looking, sociable, friendly and as having many friends. The reign of Olaf and his two brothers are regarded as something of a golden age in medieval Kingdom of Norway. Judging by the sources, Olaf seems to have gotten along with his brothers without conflict or confrontations.

The sources for information about Olaf Magnusson are Theodoricus monachus (his norwegian name would've been Tore or Tjodrek) latin work Historia de Antiquitate Regum Norwagiensium, or The History of the Old Norwegian kings, from around 1180. He was a monk at Nidaros, in the city known as Trondheim today. Morkinskinna, a saga about the norwegian kings from 1025 to ca. 1157, written in Iceland around the 12th century is another good source of information. And of course Snorri Sturlasons sagas, which in part are built on information found in Morkinskinna.

Why hadn't he been removed from the official list of Norwegian kings before 1957? You have to look at what happened in 1957: King Haakon VII passed away, and crownprince Olav was next in line to the throne. Now let's look at the list of Norwegian kings previously named Olav:

Olav Trygvasson 995-1000
Olav (Haraldsson) den Hellige 1015-1028
Olav (Haraldsson) Kyrre 1067-1093
Olav Magnusson 1103-1115
Olav Håkonsson 1380-1387

Edited, since my last explanation was wrong and clumsily written:

This presented a naming problem for the new king: Was he the fifth or sixth king to be called Olav? Since Olav Magnusson had been a child king, and his brothers had ruled for or with him, and historians had debated wheter he was a "proper" king or not, should he be counted? Experts were brought in to determine the issue, and it was decided he hadn't been a proper king since he died as a child, and King Olav was therefore the fifth king to be called Olav. (So up until 1957 Olav IV Håkonsen had been known as Olav V Håkonsen.)