Customs again to fly to Osaka?

If I'm flying into Japan from San Diego and I wanted to catch another flight to Osaka, do I have to go through customs again? I heard it might be more of a hassle to do that instead of catching an overnight bus.

Assuming this is for a domestic flight?
No customs for domestic flights, just the usual check-in.

You will have to do customs when you enter Japan from another country (e.g. leave USA and land in Japan). If your flight both departs and lands from within Japan, then it is a domestic flight there is no customs.

Where are you flying from to Osaka? If it is from Tokyo, usually the Shinkansen is much faster..

@me2 If you are flying into Narita I very much doubt rail would be faster to Osaka

Have you considered seeing if you can get up to LAX for a direct to KIX? I'm on the East Coast so trying to get to LAX or SFO to make that connection isn't a possibility, but if you can leave via LAX, you can go direct to KIX and then just need to go to Osaka. You can also do open jaw and leave from say LAX to KIX and fly back to either San Diego or LAX from Tokyo.

Last trip we wanted to start in Osaka, and while flying is faster between NRT to Itami vs. NEX to a shinkansen to Shin-Osaka, You don't save as much time as you would think because you do have to leave yourself layover time in NRT. There is also just waiting to deplane, waiting for suitcases (possibly) and getting transport to your actual destination in Osaka. In my case, I would have landed in Itami about 20:15 and then needed to get to my hotel near USJ. Instead I trained it the whole way and arrived about 22:00 at my hotel. Looking at it, you'd think I'd have saved 1 hour and 45 minutes, but the 20:15 didn't include deplaning and transport to USJ and the 22:00 train time does include getting dinner in Tokyo Station. (We didn't just get ekiben, we did sit and eat in the station.) And to be fair coming from the USA, unless you are flying Business/First Class, the NEX and shinkansen is going to feel a lot more roomy than another flight and it's easier to get up and move around.

Good luck!