Arriving to Okayama

We will land on Sunday, April 16, 2023, at three in the afternoon in Tokyo - Narita. We want To go straight to Okayama, is there a direct bus from the field to Okayama?

There is no direct bus from the airport. You would first have to head into central Tokyo. The options from there are explained here:
https://www.japan-guide.com/e/e5703.html

You can buy JR Sanin okayama area pass 4 day=4580 yen
Shimotsui loop bus in okayama 1 day=520 yen

Do you mean to fly from Narita to Okayama & are asking about a bus from Okayama airport?

Trying to check both options

The fastest and easiest way is take Narita express train to Tokyo and then take Shinkansen (Nozomi) to okayama. One merit is that you don't need reservation in advance: Normally these trains are not so crowded so you can reserve after you land in Japan. Cost: 20050 yen.

As noted by Uji, you can go by bus: There seems to be several bus companies operating Tokyo-Okayama line (night bus). You must transit somewhere in Tokyo from train (Narita express or some other trains). The bus will depart late at night so you can easily catch it. Cost:Narita to somewhere in Tokyo: about 3000 yen. Tokyo to Okayama: about 7000 yen.

Unfortunately there are no direct flight from Narita to Okayama. You must transfer to Haneda Airport to take the flight to Okayama. Cost: Narita to Haneda by bus:3200 yen. Haneda to Okayama (ANA Discover Japan Fare) 93.9 USD (about 13000 yen).

Probably you'd need to come from Narita either to Chiba station or to Tokyo station (via train), then take a long-distance bus - but do you really want to take an overnight bus to Okayama after your flight from overseas?

I would suggest taking the trains - Narita Airport to either Tokyo or to Shinagawa (also near Tokyo), and changing to JR Shinkansen to Okayama.

Is it possible to arrange the international flight to be to Osaka?

Or, add on a domestic leg.