I've been topping my chickens daily mixed greens with thyme infused acv for 8 years. I swear by the stuff. Can't prove its all the healthy stuff I feed them, but they're getting up there in age without ever having any ailments. Even the production breeds.I came across a study on black pepper that suggests that using it in over 0.3% of the diet can reduce eggshell quality, and it cites another study as saying over 0.6% garlic in the diet decreased productivity. Unfortunately, it's hard to read because English is not the authors' first language, but at least it's short.
https://www.scielo.br/j/asas/a/w8wpP6MDpF7g6QGz9NZGj5w/?lang=en
So, probably something like 0.25-0.5% (by dry weight) with breaks (something like one week on, one week off) would be a good starting point for the oregano. I wouldn't put it in the waterer unless the water gets used up in about a day. Oregano tea, like all teas I've tried, will ferment. Maybe apple cider vinegar added to the water would prevent it from going bad for longer. That gives me an idea.... oregano infused ACV... we're starting to get into salad dressing territory, here....