I grow pretty much the Dixondale Farms way, though I've grown them several different ways, including using Square Foot Gardening spacing (which I don't much like in our climate).

Here's the planting guide type info from DF:

Dixondale Farms Onion Growing Guide

In your part of OK you can successfully grow short daylength, intermediate daylength and a few of the long daylength types. Read the Dixondale Farms section on onion daylength to learn the difference. Down here in southern OK, the short deylength types grow great, but actually bulb up earlier than I'd like and that can reduce their size a bit. It isn't their fault---we just happen to hit the daylength that triggers them to bulb really early down here, but then again, that means I can start harvesting full-sized onions as early as mid-April. Any and every intermediate day length type onion I've planted here has grown and produced fine as long as I prepared the soil well and provided adequate moisture and nutrients for them. Their harvest period is more late May through late June. With the few long daylength types that I've grown here, Red River, Highlander and Copra all grow and produce huge bulbs that store very well, and most of them aren't ready for harvest until well into July.

I get the longest possible onion harvest period (from April through late July) in the years I grow all three types. The best storage onions are the more pungent ones that are long daylength types, as the sweeter southern type onions that are short day and intermediate daylength types only store for half as long (or less).

If your goal is gigantic onions, you must feed them heavily with nitrogen (the Dixondale Farms page tells you how) and they will need approximately 21" of water spread over the part of their growing period between the time you put them in the ground and the time they begin to bulb up. Once they begin bulbing up, don't water them heavily because they don't need it that late in the season---the water they get at that point won't help them enlarge and only will cause them to store for a shorter period.

The only other thing I can think of that really matters is that onions, more than almost all other veggies, do not grow well if they are having to compete with weeds, so you must keep the weeds out of their beds/rows. Really, onions are easy and most people get a good crop even with less than optimal fertlization and moisture. It is just that the more attention you pay to feeding and watering them, the bigger the bulbs tend to be.

Dawn