The only 144,000 that I am aware of are Jews saved during the Tribulation, not Old Testament believers. 
Revelation 7:1 And after these things I saw four angels standing on the four corners of the earth, holding the four winds of the earth, that the wind should not blow on the earth, nor on the sea, nor on any tree.
Revelation 7:2 And I saw another angel ascending from the east, having the seal of the living God: and he cried with a loud voice to the four angels, to whom it was given to hurt the earth and the sea,
Revelation 7:3 Saying, Hurt not the earth, neither the sea, nor the trees, till we have sealed the servants of our God in their foreheads.
Revelation 7:4 And I heard the number of them which were sealed: and there were sealed an hundred and forty and four thousand of all the tribes of the children of Israel.
Revelation 7:5 Of the tribe of Juda were sealed twelve thousand. Of the tribe of Reuben were sealed twelve thousand. Of the tribe of Gad were sealed twelve thousand.
Revelation 7:6 Of the tribe of Aser were sealed twelve thousand. Of the tribe of Nepthalim were sealed twelve thousand. Of the tribe of Manasses were sealed twelve thousand.
Revelation 7:7 Of the tribe of Simeon were sealed twelve thousand. Of the tribe of Levi were sealed twelve thousand. Of the tribe of Issachar were sealed twelve thousand.
Revelation 7:8 Of the tribe of Zabulon were sealed twelve thousand. Of the tribe of Joseph were sealed twelve thousand. Of the tribe of Benjamin were sealed twelve thousand.
Don might have just referenced them as Old Testament brothers as a nickname more than those men being something beyond tribulation saints that are also Jewish and chosen for a task. At least this is the way it comes across to me. But in the event
@DonK might have meant something else, as one new to our forum brother Don, there are certain ranges of doctrine our forum holds to but there is also given grace and opportunity to express or ask about differing perspectives to a degree...of course without it breaking out into a food fight (which being a former member of JDF I kind of miss actually...lol).
But I bring this up Don because I ended up at a church for a couple of years that were 6th seal rapturists. Something I never held to. And would debate...lol. But they were true believers. Just quite a different eschatology. At one point I wanted to meet with one of the elders weekly just to understand what they believe. Because they use similar terminology but meant kind of different things which confused me because I was not sure on some point if I agreed or not.
One of them was the 144k. Many believers see that as some form of resurrected Old Testament Saints. Some the babies that were slaughtered by Harad. And there are other views. Revelation was a book I was never all that familiar with for decades. But 8 years ago I finally decided to dive deep. So now ironically it is one of the books I am most familiar with because I had so many struggles processing it. Even so, even though very familiar, there will always be things in it where believers see different things.
My perspective used to be on the 144k that they seemed to be resurrected because of their appearance in chapter 14. And many hold that view. But I have come to view the book of Revelation to be written in what is called modular narrative. This is a writing style that developed only within the last two hundred years. And is most found in Hollywood motion picture writing. I'm not an expert on apocolyptic Jewish literature so i'm likely missing something with that take of mine. But when viewing Revelation in this way, it is clear that Revelation is written in some ways that do resemble modular narrative style. And chapter 14 to me would be one of them. So just for the sake of context, this is what that would mean in terms of its linear progression. Modular narrative does not use normal sequence of events like standard narrative. But even so, I believe the book of Revelation is written in sequential order for the most part. With some flash back/flash forward going on too.
THINGS THAT WERE
Chapter 1 Intro
THINGS THAT ARE
Chapters 2-3 The Churches
THINGS TO COME
Chapters 4-22
!ST HALF OF TRIBULATION
Chapters 4-5 (in heaven preamble)
Chapters 6-9 -- Seals and Trumpets (first half of trib)
2ND HALF OF TRIBULATION
Chapters 10-19
AFTER TRIBULATION
Chapters 20-22
To me, chapter 7 is an introduction to the 144k at the time of the 7th seal (Trumpets) approaching. The 144k get sealed during all of the Trumpets. And then have their Chapter 10 ministry for the whole second half of the tribulation (the releasing of the bowls)
To me, chapter 7 introduces them to us. But in their introduction shows the fruit of their ministry to come after their sealing. They bring many to the Lord during the second half. The reference to those not suffering from the sun heat in that chapter, to me, references those saved by the 144k during the second half where bowl 4 was scorching heat. So I would see that as a flash forward.
Chapter 10, 11, and 12 to me are a set of views resetting the reader. And orienteing the reader to the landing upon of the 2nd half of the trib by a set of tours.
Chapter 10 (Tour 1) -- From the standpoint of the beginning the 144k ministry
Chapter 11 (Tour 2) -- From the beginning of the tribulation to the the midpoint via the 2 witnesses perspective
Chapter 12 (Tour 3) -- From the beginning of the tribulation from the Celestial/Isreal-Church trade off point to just past the midpoint of the tribulation
Chapter 13 -- A focus of the midpoint highlighting events coming up to it as it relates to the Antichrist (I guess, Tour 4? lol).
14 -- A table of context for the second half of the triublation
15 -- A tribute to the second half of the tribulation
16 -- The literal rollout of the Great Tribulation (2nd half)
17/18 -- The tribulation (mostly the second half--but I believe this great city from during the escalating of the 1st half of the tribulation...so both seem to be included with emphasis leaning 2nd half-ward) from the view of Babyon the Great perspective
19 -- The conclusion of the tribulation
. . . . .
And that is what I mean by modular narrative. It uses literary devices to highlight things along the way while going in a linear line it takes some scenic route poetic license too...in my estimation.
Having said all of that I would just interject here that Chapter 14 does seem to do a unique thing in relation with chapter 15. The way I view this is that chapter 15 is an inaugural celebration of Israel as a nation through the 144k uniting with God's original purpose for them (a party in heaven). Finally happening successfully on earth. That they truly witness for Christ to the world. A big celebration in heaven. And in chapter 14 (on earth where the 144k are) we get to see them honored from heaven and with Christs celestial blessing of them on earth (chapter 14). There seems to be some interplay between the 144k on earth and the celebration in heaven of what their role in the world for 3.5 years will be. If that makes sense? But I would see the 144k as not resurrected but chosen Jews to represent national Israel being used and led by Christ in the Great tribulation. Anywayz just trying to not make it confusing...where I would be coming from in my view on that...and what I mean by modular narrative. Blessings. And again...welcome to our forum.
