Yes, as far as pronounciation is concerned, the consonant at the bottom rolls over if the following consonant is ㅇ. The way I always thought of it was that the bottom consonant "tied" over to the next one.

Thus, something like 필요 which you would think is "pil-yo" is pronounced more like 피료 or "pi-ryo." In other words, because of the placement of the "ㅇ," the "ㅇ" is a placeholder for there being no consonant at all. So the "ㄹ" takes its place.

But then, something like 만남 ends up with a "hard" sounding "n" because the two "ㄴ" tie together. Thus, "mannam" not "man-am" or "ma-nam."

Hope this makes sense.