The Culture explores 40K II: Now With 100% more Fanfiction

[Tiki Snakes]

In the short term, to study their society and be the ruler.
In the medium term, use as weapon against Chaos.
In the long term (with genetic engineering since it'll be deciphered by then), reform and civilization.

A slow genetic drift over a very long period (a few centuries) might be enough to get the orks to start becoming more civilized. You start by reducing their aggressiveness by a small tolerable level, then once all the current orks die (which won't be too long), you lower it again.

That's the plan for Large Sticks Speaks Softly. Some parts of the Culture might say that's making an entire new race, but Large Sticks Speaks Softly hopes to preserve some of their culture through the transition to make a race that can stand on its own.

I'm not entirely sure on this, but there's a distinction to be made here. It's not strictly speaking that Orks are aggressive. Berserkers of Khorne are aggressive. Orks just love a good fight. It's what they're about. I'm not sure that's something that could be changed. Not without eliminating Gork and Mork, and/or wiping out all existing orkoid life-forms in the galaxy first. They aren't, after all, an exclusively biological entity.

Some of the rest of that isn't too far fetched though. It varies.
Use them as a weapon against chaos? That's not mid-term.
"Oi, you lot. We'z off ta fight da pointy boyz!"
"WAaaaagh!"

That's all that takes.

But, yes. Orks don't rampage through the galaxy because they're angry, or because they hate everyone, or because they believe that Orks are the natural inheritors of the galaxy. They do it because Orks is made for Fightin' and Winnin. And actually, that should come across relatively quickly to the Agent. The Orks, when pressed on their opinions of the other factions would pretty much exclusively describe them in terms of how good a fight they are. No malice, (well, maybe a little if a faction is particularly well known for being cheatin gitz), no spite, no real urge for their destruction, just the constant urge to have a good fight with someone. Anyone. Everyone.

If he wants to make them, long term, into less of a destructive swarm, he'd do better trying to redirect that theme than to limit or mitigate it.

Edit: Oh, and it's probably worth remembering that there is often a thing in similar situations where enough exposure to orks and orkyness rubs off. This is played up in fanfiction more than in canon, I'm sure, but even in canon there are some hints that it could be a thing. There's a very, very real chance that if they hang out with the Orks long enough, and especially if they do what they have to in order to come across as Orky enough not for the whole thing to fail instantly, that even if they did manage to adjust the Orks, there would be a degree of meeting in the middle. Partly because of potential warpy-weirdness with the Orks and their gestalt field, partly because there is an element of genuinely honest truth in their outlook on the world. The Orks are the happiest and most honest and most joyful of all factions in the galaxy. Their philosophy is simple and more than a little persuasive. It's exactly the kind of thing that the Culture wouldn't expect to be effected by, but over a long term, I'd expect there to be a good chance that the Agent and Loud Stick Speaks Softly to take on some shared traits.

Like, a focus on winning (which the culture in their arrogance already have a bit of a vulnerability to), a belief that because they have the power to, that they should (which the culture in their arrogance already have a bit of a vulnerability to), the idea that brute force or cunning can be used to solve their problems (which are the two methods the culture already uses. Either superior dakka or sneaky gittishness.)
It would be slow, subtle, and not inherantly a bad thing if the culture succeeds in affecting their Orks for the better at the same time. It's not in the same bracket as Chaost infection either, mind you. It isn't as wibbly-wobbly, nor as dangerous in the grand scale of things. It's very much something to remember though, especially as the Agent's plan involves pretending to be an Ork for a few hundred years.