I think that comparing Oscar to Zero is apples to oranges because the Zero is a carrier fighter that serves as both a CAP fighter and as an escort. There are other factors to consider such as altitude. Naval air battles are somewhat restricted in altitude and often occur at lower altitudes than LBA battles.
As for using air in support of ground operations, the issue is not so simple as you present. With or without escorts, any type of bombing will be very costly in a theater where you are at a significant disadvantage in numbers. If you cannot achieve a numbers advantage, you will soon be unable to fly any offensive missions: with or without escorts, because all of your planes will have been destroyed. Flying into heavy CAP will get your escorts destroyed, and you will not be able to do this often. In order to bomb a target, you must sweep that target, which means that you are not limited by the range of your escorts. Rather, you are limited by the range of your sweeping fighters.
Oscar is a very poor sweeping fighter. It is slow, has two machine guns, depends upon manueverability for survival, does not climb very well (especially compared to Tojo), and has a very low effective ceiling (because it depends on manueverability for survival). The two machine guns can be effective against slower allied aircraft, but the Oscar lacks the speed to sustain bursts at close range, which you can see in the air-to-air replay. What happens is that Oscar damages planes, but gets destroyed in return. The inevitable result is that the Japan loses air superiority in theaters where only Oscar is present, and offensive air operations become impossible at any range.
Tojo is fast, has 4 machine guns, and a much higher effective operational ceiling. It can bring its guns to bear and get kills. (It is also a very good, even excellent, CAP fighter.)
Sorry, too many inaccuracies here. I have to speak up. I refrained earlier ...
The first Tojo gets 2x7.7 CL, 2x12.7 F. The Oscar has 2x12.7 CL until the IV model. These are equivalent armaments. 2x12.7F = 1x12.7 CL literally. The accuracy for CL is doubled. Comparing 2x7.7 CL and 1x12.7 CL, roughly the same, many players will give the edge to the 12.7 but most would agree it is a close thing.
Next range. The first Tojo will sweep at 5 hexes, for DOUBLE your supply you get 6 hex. Oscar is also 5, but for double your supply usage you get 10 hexes. If you plan to sweep at extended range, you will learn the penalty for that quickly.
All the other factors are mitigated by how you setup your missions (altitude, leaders, etc.). If you don't agree, I suggest you go read Herb's last long AAR, he is an acknowledged expert in Air Combat and he detailed his mission prep routine thoroughly. His results are spread throughout 3 of the best written AAR's on the forum. Except for DUR. That is the real difference between the two. Oscar 22 DUR means armor is almost moot; for the pilot to survive, the aircraft has to make it home, Oscar doesn't do that well (it tends to disintegrate . Tojo 28 DUR gets the AC home more often, you lose the AC (crash on landing), but save the pilot. In the early war, losing a pilot is a big deal.
After about 3/42 though, unless the SOV are activated (Lowpe!), you should never, ever be short of fighter pilots. If you are, you need to work on your pilot training program(s). What this means is that pilot losses are also almost moot for the IJ by the time the Tojo is available.
If you practice good mechanics for your air missions, you should see very little difference between Oscar and Tojo in sweep effectiveness with two caveats:
- slightly higher AC losses and noticeably higher pilot losses due to DUR for the Oscar
- far greater flexibility in terms of basing and a higher tempo of operations for the Oscar.
While the IJ has initiative, IJ has little problem making up AC and pilot losses and can always achieve positive force ratios (much like the allies from '44 on) when she wants. Initiative is crucial in combat and this reality is accurately modeled in this game. The 2nd caveat should be of greater import than the first for IJ players until they cede the initiative, and that should be as late in the game as possible. This means, you should be using Oscar primarily with Tojo being your preferred CAP fighter.
Once you get Frank, you have a multi-role fighter that is now the best in both roles for the IJA. This is why the emphasis on Frank. Getting Frank in late '43 is a BIG deal, although not as big as getting Sam in mid '44.)