Winston Smith's work at the Ministry of Truth is to rectify or update the historical record so that it corresponds with previously published newspaper articles. A case in point is Comrade Ogilvy. He is entirely Winston's creation, invented to replace Comrade Withers, an Inner Party member whom Big Brother had awarded with the Order of Conspicuous Merit, Second Class, but who has since fallen into disgrace and been vaporized. Rather than complicate Big Brother's panegyric, Winston decides to lift Withers clean out of history and replace him with a fictitious character, Ogilvy. For this reason he is a flawless patriot and Party member whose 'only goals in life were the defeat of Eurasia, and the hunting down of enemy spies, saboteurs, thoughtcriminals, and traitors'. In fact, Ogilvy, who did not exist in the past, but is called into existence in the present, can be considered a caricature of the world of Big Brother. And herein lies the significance of Comrade Ogilvy. His world, where a man can disappear from history, but a fictional character can appear in history at the stroke of a pen (or Winston's speakwrite!), is above all else stamped with arbitrariness. In this world, reality is no longer written about. Rather, it is the written record - at the behest of the Party - which creates reality.
In Part One, Chapter Four, while at work, Winston is tasked with writing a man called Comrade Withers out of history. (Withers has been vaporised and is now considered to be an enemy of the Party). To do this, Winston creates a substitute figure called Comrade Ogilvy. This man has no factual basis whatsoever; he is completely drawn from Winston's imagination but, by writing him into history, he has come to life.
Ogilvy is a model citizen of Oceania and the perfect Party member. As a boy, he was a Troop Leader of the Spies, for example, and he reported his own uncle to the Thought Police. He does not smoke or drink and his only conversational interest is the "Principles of Ingsoc." Rather ironically, Ogilvy is the complete opposite of his creator, Winston, who is already beginning to rebel against Party rules and doctrine.
As a result of Ogilvy's existence, Comrade Withers has been written out of history and, as such, has become an "unperson." But this does not matter since the Party controls information and can rewrite history whenever it chooses. Ogilvy's significance, then, is exactly this: he shows just how easily the Party is able to control the past, present and the future.