Narita Airport to Oimachi station?

I'm travelling from Narita Airport to Oimachi Station.
I plan to take the Narita Express to Shinagawa station and change to JR Keihin Tohoku line for Oimachi station.
My question is, does the Narita Airport Express ticket machines at the airport, sell a single ticket to Oimachi station?
Or do I need to buy separate tickets from Airport to Shinagawa, then Shinagawa to Oimachi?

Generally your Narita Express ticket will take you all the way, if you don't have to pass through any exit gates. So the N'EX ticket to Shinagawa will cover you.
The N'Ex comes in at platform 13 (I think) you take the escalator or elevator up to the concourse area, head towards the sign saying Central Gate/Takanawa exit, but DON'T go through the gate, stick to where the shops are, and head back down the escalator to platform 5, which I think is Keihin Tohoku Line. Get the train heading to Oimachi, and when you get off there hand your N'EX ticket to the guy on the exit gate. They usually don't ask for any more money. It's only if you post your N 'EX ticket into an exit gate that you will have to buy another for the second train. Check out a map of Shinagawa station and you will see what I'm trying to explain.

Generally your Narita Express ticket will take you all the way, if you don't have to pass through any exit gates.

Generally, no, it won't. In this particular case, it will, but that's because Shinagawa to Oimachi is short and does not make a difference regarding the fare (the fare from Narita to Shinagawa or from Narita to Oimachi is the same at 1,490 yen). If OP were going, say, to Kamata, the fare is not the same (Narita to Kamata is 1,660 yen) so a fare adjustment (170 yen) is required. Trying to sneak out of paying the additional 170 yen would be fare escape and illegal.

It is not the only way to go to Oimachi stn fr NRT, but I will not write that info here.

The Oimachi stn is also a rail company JR-EAST's, you can buy tickets at vending machine.
However, the "base fare" to Shinagawa stn is also same 1490Y/oneway adult,
but it does not become valid to the stn Oimachi, this is a first important point of notes.
And, since N'EX is a Limited Express(LtdExp) train, it can not be taken with a base fare ticket,
so "(reserved seat) LtdExp ticket" is also necessary, 1490+1700 = 3190Y/total.

Anyway, if you do not buy discounted one in the fifth E), you can buy it in the following 5 ways.
A)
At multiple vending machine base fare and LtdExp tickets both separately.
https://youtu.be/PbS0J-2UIYw
You can pay by "Suica" card that mentioned at C)D) instead of cash(JPY),
and only by these major brand credit cards also possible.
https://www.jreast.co.jp/mv-guide/img/card-ichiran.png

B)
At "JR EAST Travel Service Center" or "Midori no madoguchi =ticket office". (easiest, fastset)
https://www.jreast.co.jp/e/ticket/purchase.html
(note)
JR EAST Travel Service Center's business hours is shorter.
And NRT has 3 terminal buildings but rail stn is 2 only, for T1 and for T2/T3.

C)
Use IC card ticket "Suica" instead as base fare, buy LtdExp paper ticket at vending machine.
https://www.jreast.co.jp/e/pass/suica.html
Suica is not big discounted ticket but very convenient also as e-money for shopping and eating.
https://youtu.be/G9ZaZbSjzn4

D)
Use Suica that mentioned at C), and LtdExp paper ticketat ticket at ticket window of B).

E)
Buy a discounted one that can be used for N'EX also covered until Oimachi.
https://www.jreast.co.jp/e/pass/index.html
N'EX TOKYO Round Trip Ticket, JR TOKYO Wide Pass, or. (depends on yr travel plan)

but it does not become valid to the stn Oimachi, this is a first important point of notes.

What do you mean, it does not become valid? The fare is the same, so at most you go to the fare adjustment machine and pay an extra zero yen...