Organic cotton denim used to mean something stiff and virtuous looking, the sort of jeans that told you they were good for the planet before they told you anything about how they'd sit on a Tuesday. That's shifted properly in the last few years. The better organic cotton jeans now have enough stretch woven in, or enough give in the weave itself, that you'd struggle to tell them from the conventional pair next to them on the rail, and that change has been entirely for the better. Nobody should have to choose between their conscience and a waistband that actually moves with them.
Before you buy, get your hands right into the fabric and check the weight, because organic cotton denim can swing from a light, almost shirting weight to something properly heavy and rigid, and the label won't always tell you which. Look at the rise against your own torso rather than the size chart, since organic cotton has less of the synthetic stretch that forgives a bad cut. Check the stitching at the pocket corners and the inside of the crotch seam, because that's where a cheaper finish gives up first, usually within a few months rather than years. And feel the inside of the leg. If it's rough enough to notice through tights, it'll be rough enough to notice through everything else too.
Where this fabric struggles is next to anything slippery or synthetic worn close to it, a satin slip skirt layered underneath, a nylon liner, that sort of thing, because organic cotton has more texture and less glide than you expect, and it'll cling and bunch rather than sit. Darker indigo and a true black wear the most easily over time, hiding the fade that cotton does naturally as the dye works its way out with every wash. A mid wash is lovely to look at new but shows that fading unevenly, especially at the thighs and behind the knees, so go in expecting that rather than being disappointed by it.
Over months, organic cotton softens properly, more than most denim, and it moulds to how you actually sit and move rather than holding its own shape. That's the trade off for that initial stiffness. Wash it inside out, cold, as little as you can manage, and let it dry flat or on a line rather than in a tumble dryer, which is where I've watched a good pair shrink two inches overnight and never quite come back. Looked after properly, they'll outlast most things in a drawer.
The honest limit is that organic cotton jeans will never have the recovery of a pair with real stretch fibre in them, so by the end of a long day they'll have given a little more than you'd like, and they won't snap back.