Opportunity and Visibility

How do folk apply visibility limits to opportunity shots and reactions? Had a situation in a recent game where a gentle hill restricted visibility for shooting, so obviously an "opportunity shot" was not allowed. But what about an "opportunity charge"? (the base on opportunity was Scythian LH, comp bow B and impetus 1). We decided that you cannot react to what you cannot see, and so disallowed an opportunity charge. But now I am not so sure, as you can declare a normal charge against a technically out of sight target.

Views?

How do you declare a normal charge on an out of sight target?

5.8 says the following, "You cannot charge Units that you cannot see (maybe hidden by terrain elements (3.1.3))."

Well spotted! Missed that one...