They NY times said senior trump official wrote the op-ed. So it's clearly one of the closest officials to the president.
“Senior official” is so purposefully vague though. It can refer to someone in the WH that’s visible and directly works with the POTUS, or it can be a top official in any one of the number of agencies.
The op-ed also infers that there’s some kind of coalition, that it’s not just this one senior official, therefore it’s some kind of grouping of people. That’s what made me think it’s probably intelligence agency personnel, or something of that nature (our intelligence community has been highly antagonized by Trump, and it’s their job to keep the country safe).
Scary times..