Bob’s Red Mill won the Golden Spurtle for their steel cut oatmeal.
That’s the opposite of what the OP is looking for.
I have the same issue. My solution is easy: make it ahead of time and let it sit out on the counter for an hour after it’s cooked. It’ll settle down and solidify a bit, and then you can slice it cleanly and easily and the slices will hold together just right. If it’s cooled down too much, put your plate with the slices on it in the microwave for thirty seconds or so, and they’ll reheat back up just right.
Posting, having only read the OP.
This is the recipe I use:
- 1 envelope Lipton Onion Soup Mix
- 2 lbs. ground beef
- 1 lb. ground pork
- ¾ cup plain dry bread crumbs
- 2 eggs
- ¾ cup water
- ⅓ cup catsup
Combine, and divide into two bread loaf pans, one lined with plastic wrap. Tightly wrap the plastic wrap around one loaf, freeze, and transfer to a one-gallon zip-top bag for future use. Bake in a 350ºF oven for an hour or 90 minutes. Top with tomato sauce for the last 20 minutes.
No worries about the texture, either cooked right away, or defrosted and baked later.
I made a double batch last time, and froze all four loaves.
How is that the opposite of what the OP is looking for? They’re asking for tips on a less crumbly meatloaf. I made the point that meatballs, which are essentially little meatloafs, are often made with a panade, which is a mixture of bread and milk that bonds with the ground hamburger to form a more tender, less crumbly texture that holds together better.
Meatballs win me over every time.
My current favorite recipe flaxseed meal
I like Mezzetta marinara
She wants a firmer meatloaf.
This tracks. 
Less liquid don’t steam soak the loaf.
Maybe I’m mis-stating the terminology here re: the word ‘tender’. The OP did mention firmness, but more in the context of not being crumbly and falling apart when sliced. Adding a panade to the meat creates a texture that’s tender but not falling apart crumbly.
Here’s a passage from the article about panades that I linked to upthread:
A panade is a mixture of bread and milk, combined to form a paste, that is incorporated into ground meats so that they don’t get too hard or dry when you cook them.
When you cook ground meats, the proteins contract and squeeze out moisture. You’ve likely noticed this when you’ve seen a burger on the grillshrink and change its shape during cooking.
Starches, on the other hand, absorb liquid when they’re cooked, so they expand, and undergo a change called gelatinization. Think about what a bowl of oatmeal does if you let it sit for too long—it goes from being a thick but still pourable liquid into a jiggly, semi-solid, bowl-shaped mass. That’s gelatinization of starches.
So when you combine a panade with ground meats, the starches interfere with the tendency of the proteins in the meat to curl up when cooked. Meanwhile, the gelatinization, which is caused by the starches absorbing moisture, helps ensure that the final product isn’t too dry. It’s basically like adding little milk-soaked sponges to your ground meat.
I’m with @solost … one path to a less “crumbly” meatloaf would be more bread/panade or whatever non-meat filler you’re using. Sounds like oatmeal is something common for folks? I’ve never had or used that. I’m also wondering if soaking the oats in advance might help (or maybe that’s already happening)?
Hadn’t tried soaking the oats, just pour them from container to mixing bowl, but with that discussion of gelatinization, makes sense. Maybe the “mix the day before, refrigerate overnight before baking” strategy would double as soak time?
Bread, or probably oats mixed with milk (or some other liquid) I think is the key, before adding the panade to the meat. I don’t think it has to sit very long, just the process of mixing the oats / bread and milk / other liquid first, maybe letting it sit for a few minutes to bind, before adding to the rest of the ground beef mixture will work.
Forget the oatmeal, use bread crumbs or ideally, partially crushed croutons. I like to keep a bag of the Franz bread croutons that are always on sale around Thanksgiving. Use those or large bread crumbs or make you own out of bread toast.
With oatmeal you are not getting the glue that holds the meatloaf together.That glue is the gluten, the sticky part of bread.
If you want it to hold together use flour instead of the oatmeal. You need a combination of eggs and gluten to get there. Adding an egg won’t hurt either, since that’s a fairly large meatloaf.
I do find that mine fall apart the way you’re talking about if I put too much filler in it, like the oats you used. I use corn flakes or dry stuffing mix or other types of non-sweet cereal and if I overdo it it falls apart. The amount of oats you put in might have been too much.
I’m always tempted to use two packets, but I only use one. Dad used to add a quarter cup or so of dehydrated onions. I don’t. I use three pounds of meat and only two eggs, but they’re jumbo eggs.
I’m just cooking for the two of us and and apparently making a different sort of meatloaf.
I usually have 1.25 # packages of ground bison in the freezer. One egg. One chopped onion. Some Worcestershire sauce. Some garlic. Half a can of chopped tomatoes. Enough seasoned breadcrumbs that it holds together.
Plop it in a flat pan mounded up in a loaf shape. Mix the remaining chopped tomatoes with ketchup and slather over. Bake, maybe 45 minutes. It’s not fussy. It holds the loaf shape fine.
Clean bread like slices it does not produce but very yummy with mashed potatoes and a vegetable. If I wanted firm I’d cut the onions finer and minimally sweat them down first. There’s always plenty leftover when cooking for two.
Last time I made meatloaf, I only used the loaf pan for forming the loaf. Lined the pan with parchment paper then flipped over onto a flat pan, also covered with parchment paper. I used the recipe in the Better Homes and Gardens New Cookbook, copyright '96.
Had to double the amount of eggs and breadcrumbs due to the amount of ground meatloaf mix being slightly larger than what the recipe calls for but it turned out well.
If it don’t hold it’s shape call it meat pie.