Oh boy, my favorite.

Nobunaga didn't have a lot of things going for him initially. Owari had a few advantages: namely, its central location made it a center for commerce and trade, and it had relatively fertile land. However, this was also a big disadvantage, as it made it far easier to be attacked from multiple sides.

One of his first policies once he had unified the province, however, was something called rakuichi rakuza, or 楽市楽座 (lit. "Ease the standing of the cities"). Merchants were looked down upon by many of the feudal clans that held power in Japan. Nobunaga's policies here expanded commerce in Owari, as many merchants headed there to take advantage of the easier commerce regulations and policies. It is sometimes cited as one of the earlier examples of free-market policies, although it was really overtly dominated by the Oda clan. This however gave Nobunaga a large amount of economic power, allowing him to mobilize and equip a relatively large force.

This expanded trade in turn led to the arrival of European traders and missionaries. Nobunaga was, according to the missionaries, an atheist who would nonetheless pave the way of God in Japan. However, what Nobunaga was really interested in was firearms. The firearms that were introduced gave him the ability to expand the fighting ability of his men, as he was able to quickly raise troops by training them how to use firearms (which are easy to train troops to use), while at the same time having the economic power to maintain such an army.

The third thing that helped Nobunaga was sometimes just plain luck. At Okehazama, his first great victory, was due to a surprise attack that he was able to pull off against a much larger force. Similarly, two of the warlords that could have threatened him, Takeda Shingen and Uesugi Kenshin, died of various causes before they were in a position to directly undermine him.

Lastly, and I would argue was probably the biggest contributor, Nobunaga was born in the wrong time period. Simply put, many of his actions were, if not quite anathema, highly unorthodox to any prior Japanese tradition. His welcoming of foreigners, for instance. His rather ruthless reputation (Demon King of the 6th Hell and his reputation of turning his enemies' skulls into tea cups comes to mind), his puppeting of the Shogunate and his overall attempt to unify the land under "Tenka Fubu" (Unite the world under military rule), his choosing of subordinates (Toyotomi Hideyoshi being an obvious one), and so forth. Yet it was this sheer non-conforming nature that gave him the ambition to try and rule Japan, to defy the odds where most others would have given in.

Source:

Lamers, Japonius Tyrannus,

Mason, A History of Japan