I'm not about to say you're doing it wrong (Oh dang I just did) but for the 3rd time, the Druid I ran 5s with was solo healing the entire team, and not going oom.
My shaman would all be completely empty, and he would still have 80-100%
The only thing that's going to oom a skilled Druid healer is a priest (or lock) successfully free casting Mana burn (Drain).