Bought tan oak flooring from local manufacturer for custom house. Acclimated in the house for a month, installed, finished, moved in.. year later amd we have huge cracks/gaps between lots of the planks. 1/8th inch all over the place, probably every 6th board.. some gaps bigger, 3/16” in bad spots.

Any advice on what can be done? Lots of different colors in the planks. Afraid filler will look bad, but no idea what else to do.. will filler even work on gaps of this size?

Hopefully don’t have to rip it out and start over. I’m guessing some on the planks were either never put in the kiln, or say around for 15years since and soaked up moisture.

It's too bad that the flooring has shrunk. Just leaving it in the house for a while doesn't always work since it takes time for moisture to evaporate. A moisture meter is imperative for a floor installation. If there were a few boards that have gaps between them you could add some tapered splinters to glue in wide gaps.
With most of the boards showing gaps between, I would consider removing and re-installing. You would need to have extra wood to cover the floor and some extra to cover broken boards and cutting waste, unless you marked each board to put it back in the same row, since oak doesn't shrink in length that you could notice. Unless it is glue down, re-laying the entire floor is the best course.

Unfortunately the moisture meter I used was junk… so I’d say use a good moisture meter next time.

How do you pull it up with out destroying all the tongue and grooves? Cut the nails?

Thanks

Starting at the last boards laid, try to get a pryer under the last row. If it is too close to the wall you will need to use a circular saw and rip it full length. A chisel will help get it.
After you get a few rows out, a diamond grinding wheel in a 5 inch angle grinder will be able to cut the fasteners off at the surface, or below, which would be better. You don't want to have any projections which will scratch other boards as you stack them.
It's going to be best to collect the boards by length taping them together in groups of 12, by length. Aces, duces, threes and so on until all lengths are collected. This will help with installation. Keep track of how many bundles of each length you have. As you get ready to start installing the flooring you will have totals of all lengths so you can spread them out in each room so the butt-joints are random instead of running out of long boards and having to use all short boards near a wall. If you do plan for installation in a closet, use short boards there.
It's a lot of work just to get the flooring laid again, but you aren't done, yet. You will need to sand the floor again because not all the boards were not sanded evenly, depending on where in the room they were located.
You may decide to cover almost all the room with area rugs accepting how it looks, now.
I've seen floors like this covered with carpet from flooring left in poor storage.
Hope you can afford to spend the time.

If hiring a flooring guy.. Would it be cheaper to grinder the fasteners and pull it all up carefully… vs laying new flooring?

It may be cheaper to get new flooring and then a contractor to install and finish.
It all depends on how much you paid for what you have and the cost of new flooring.
It doesn't take long to remove the flooring and then grind the fasteners. Grinding the fasteners is done after removal. It is work that lower paid handyman could do if they are in good shape. Two people working would help since the fasteners on a long board could be loosened better. Almost all the flooring could be re-used once the technique is mastered. Just one row removed at a time.