This is a great question. Combining oat straw and milk thistle in a care plan for this case sounds like a good idea. The nervine effect of oat straw (or seed) will not disrupt the milk thistle.
I’d recommend offering them in separate forms:
Milk thistle: The liver supportive constituents in milk thistle are not water-soluble. So, that means an infusion will not be effective. Rather, milk thistle is best taken in powder form in capsules or sprinkled on food. A tincture is another option, but of course not recommended for someone working with addiction issues.
Oat: The oat is great as an infusion! For this case, I’d specifically recommend milky oat seeds if you can access them. They have a bit of a stronger restorative and nutritive effect. For working with the seed, you want to do an overnight infusion for optimum extraction. I also like to pulse the seeds in a grinder to break them up a bit and speed up the infusion.
I hope this information helps! Let me know if you have any more follow up questions.