What is the proper way to bake meat balls in the oven?

Techniques I use for tender meatballs:

• Breadcrumb slurry - mix breadcrumbs and milk (or cream) into something that's almost a paste. Mix this into your meatballs with the egg and spices. I use a small bowl per butcher's package of ground.
• Low and slow - cook the meatballs in sauce in the oven or slow cooker at a low temperature (making sure to hit the minimum internal temperature required for the grind you're using).
• Add fat - sneak a few spoonfuls of bacon fat (or duck fat) into the grind, in addition to the breadcrumbs. Alternatively, mix in a fattier cut of meat (or hand grind good, tender cuts). Cheese can add flavour and fats too.
• Fast fry - if you have a tender grind, a fast fry (or broil) can minimize the loss of moisture.

One technique I have used is to support the meatballs with toothpicks over a muffin tin. This allows the liquid to run off and collect. I forget which food network show I learned this from, but it has been a good technique for me.

I'm trying to minimize the outbound loss of fluids

As fat is rendered to liquid it has to go somewhere. It is either going to be absorbed into something (like dry bread crumbs) or it will leave in the form of 'juices' oozing out of the meatball. I have to disagree with @Bruce's "breadcrumb slurry" as that adds an additional saturated element that leaves no place for fluids to go (other than to leave). Dry breadcrumbs will absorb those juices before they escape. One way to put that juicy goodness back into your dish is collect those juices, mix with some butter and an equal (total) amount of flour to create a rue and then either add milk to that to create a cream gravy to serve over the meatballs or add it to a tomato sauce to thicken the sauce in infuse the meaty/juicy goodness to the sauce and so it will stick to the pasta :o).

I normally only make small batches so pan frying is my primary technique.

My sister-in-law has two since she regularly makes larger batches. Both rely on a fast fry to firm the surface. Then she either cooks them in sauce for a longer period at a low temp. For naked meatballs she bakes them low and long with a cookie rack in a casserole or cookie pan. I don't think it matters which because I have seen her do both.

In either technique the meatballs are ready to serve or freeze when done and fit nicely into OAMC.

I wouldn't worry about the liquid. If you pan-fried the meatballs, I bet that the same water that oozes out just evaporates from the pan surface instead.

If you've ever cooked wild game like venison that has not been deliberately brined to pump as much moisture into the meat as possible so it can be weighed, labeled, and sold at a higher price, then you will notice quite a difference.

I just tried this new recipe for meatballs that has you bake them and I lost very little liquid from them. The new ingredient in this recipe was plain Greek yogurt. My boyfriend absolutely loved them! I don't mind sharing the recipe if you would like to try something new!

I like to add Panko bread crumbs to my wet mix then mix well with the ground beef then I put them on a broiler pan that is likely sprayed with olive oil then I bake them at 300 degrees for 30 minutes and then I put them in a large Dutch oven with sauce in it already and once they are all in I cover with sauce and put in the oven 200 degrees for 3 hours they come out tender I make gravy or sauce you come from has the rich meat flavor and it's thickened my mother who is Italian and it's an excellent cook mine are the best she's ever had