Sounds like vent gleet. If it is only mild try adding a teaspoon of ACV with mother ( available at woolworths ) to the water. It will help lower the PH level and I give mine some Greek yoghurt , probiotics.
If you are going to feed rolled oats it's best to soak them in hot water. If anyone gorges themselves on them you may risk crop impaction.
https://www.backyardchickens.com/a/vent-gleet-aka-nasty-chicken-butt
I agree with Fancy. When you are looking for acv in either coles or Woolies you need to get the one from the health food isle. It has all the good stuff in it. The one they sell where all the vinegars are etc doesn't.