Admittedly I don't know a whole lot about the subject, and our actual specialists in this sort of thing can feel free to slap me, but the narrative that you've given feels like it misses a few steps because it jumps directly from Okehazama to the march to Kyouto. Okehazama was fought in June of 1560, but Ashikaga Yoshiaki did not invite Nobunaga into Kyouto until 1568, a gap of eight years. During that intervening time the Matsudaira defected to the Oda, Oichi was married to Asai Nagamasa (securing the west and the passage to Kyouto), and Mino province was taken from the Saito. The campaign on Kyouto also wasn't just a matter of marching in, Nobunaga had to launch a campaign on the Rokkaku in southern Oumi first

u/XenophonTheAthenian gave the political side of things.

Just to add the economic side of things, according to Toyotomi Hideyoshi's land survey, Owari is 570 000 koku. In comparison, Suruga is merely 150 000, Totomi 255 000, Mikawa is 290 000.

This means a united Owari province of the Oda had more economic power than Imagawa's power base of Suruga and Totomi combined (570k vs 405k) so when Imagawa Yoshimoto defeated Oda Nobuhide (Nobunaga's dad) and took over control of Mikawa, Imagawa was actually the economic underdog, and got to where they were thanks to Yoshimoto and his talented retainers. Even with Mikawa (the Matsudaira) as vassals, the total koku controlled by Imagawa Yoshimoto was 695k vs Owari's 570k. Not what you'd imagine from the normally touted 25k troops vs 5k is it? Now that's not completely accurate as the Imagawa had taken south eastern Owari, but you get the idea.

It also explains why Tokugawa Ieyasu can take on Imagawa Ujizane (Yoshimoto's son) by himself. Mikawa alone is basically equal to Suruga + Totomi.

What about Mino? Mino is 540 000 koku. This explains why even with huge internal instability in Mino it took Oda Nobunaga until 1567-68 to have the province subdued.

But once done, and with such a loyal ally as the Tokugawa Ieyasu, Nobunaga had almost 1.5 million Koku under his control. Takeda Shingen and Uesugi Kenshin at their height had only about 1 million apiece (and not at the same time). At about the same time he goes to Kyoto, he also adds in Ise's 567 000 koku, giving him over 2 million going into the 1570s. If you wonder why he could expand so explosively like he did after Mino and going to Kyoto, that's why.

So after the first/second (depend on how you count it) coalition against him collapsed, the economic power disparity wasn't even close. At that point, it was his game to loose (which he totally lost by getting killed in a coup). And really, going by this map of the first coalition, it wasn't that close either.

I'm not going to go into details, but Nobunaga, unlike his popular portrayal in fiction, is actually also a shrewd diplomat.

To further add to u/XenophonTheAthenian and u/ParallelPain, following the successful occupation of the capital, the Asakura grew weary of Nobunaga's growth and declared war on the Oda. The Asakura, who had an alliance with the Azai, convinced Nagamasa (head of the Azai and brother-in-law to Nobunaga) to join them. However, in 1570 at Anegawa, an Oda - Tokugawa alliance smashed the Asakura-Azai alliance giving Nobunaga almost unchecked power in central Japan.

Another major battle that aided Nobunaga's rise and demonstrated his tactical acumen was at Nagashino. Shingen, who had been the leader of the Takeda clan and was considered one of the geniuses of the time, had died in 1573 leaving the clan in the hands of his son Katsuyori. In 1575, Katsuyori led the Takeda and it's famed cavalry to the plains of Shitaragahara in Mikawa and laid siege to Nagashino castle. Tokugawa Ieyasu called on the Oda for help and Nobunaga led his army to meet with the Tokugawa forces. In June of the same year, the two armies met in combat. The Takeda were renowned for the shock-cavalry tactics perfected under Shingen, and Katsuyori was planning on using this against his enemies; it would be against this cavalry that Nobunaga would demonstrate his brilliant understanding of technology and war. Nobunaga, anticipating the cavalry charge, employed 3,000 arquebusiers in a pattern that allowed for repeated fire (one loads while another shoots). This coupled with Nobunaga's long spear men obliterated the Takeda cavalry and effectively neutered the Takeda army as a whole.

The final addition I'll add is that Nobunaga was very fortunate when it came to generals in his employ: Toyotomi Hideyoshi, Akechi Mitsuhide (an asset turned liability), Shibata Katsuie, Meada Toshiie, and successful alliance with Tokugawa Ieyasu were major boons to his rise.

Source: The Samurai Sourcebook by Stephen Turnbull

Edit: a word.