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.
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
At "JR EAST Travel Service Center" or "Midori no madoguchi =ticket office". (easiest, fastset)https://www.jreast.co.jp/e/ticket/purchase.html
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.
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
Use Suica that mentioned at C), and LtdExp paper ticketat ticket at ticket window of B).
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)