Well yes, Oda only became an activist after the war, but don't believe that public opinion in Japan during the war was so monolithic. History is never, never that simple, and I hope you remember that!

While I don't know too much on the subject, I would say the Japanese socialists/communists would have posed some opposition to what they saw were imperialist wars. By the time of WW2 though, most of their elements were driven deep underground as a result of the government crackdowns on left-wing groups. So, while there may have been some small number that were openly opposed to the war, they were most probably imprisoned or executed immediately.

Aside from that, I thought it'd be worth noting before the militarisation of Japanese society that led to the Pacific War, the 20s saw a flourishing of the liberal democratic movement in Japan. Along with this was the emergence of liberal intellectuals like Minobe Tatsukichi, whose work on the role of the Emperor suggested that "the emperor’s right of supreme command over the military needed to be carefully limited by the Diet of Japan if Japan were not to end up with a dual government in which the military would become completely independent and above the rule of law and unaccountable to civilian authority". He and many other academics and politicians were attacked during the 30s, with their works being officially discredited and banned in favour of those advocating emperor deification and right-wing nationalism.

So, we know that there were Japanese who openly protested against the wars; they were locked up and killed. We know about the Japanese intellectuals that were silenced because they wrote critically of the Emperor or the militarist government. On the level of the ordinary Japanese people, I have no doubt that public opinions would have varied tremendously, even as they went along with, and were complacent about, everything the government proposed. I can imagine that doing anything else would have felt futile at the time. Again, from Oda Makoto:

[A]t the end of the war, Japanese people were supposed to weep. Our family didn’t weep at all. Many people didn’t weep. Look at those pictures of people crying in front of the [Tokyo imperial] palace. But count the numbers: It’s so small, 100, 200, something like this. [laughs] So the entire population didn’t weep, so exhausted we were. My mother’s reaction was the typical Japanese common people’s reaction: "Oh, they started the war and they ended the war."

Even the Kamikaze pilots, who are often remembered to be the most fanatical of them all, each held very different views about the war, their government, and why they were giving up their lives: Source.

I suppose I'm just trying to highlight that in no modern society has there ever been "no dissention or debate on the matter of war". Even the lack of sustained opposition would not mean that all Japanese believed in the same single thing. Societies, and human beings, are much, much more complex than that.