Hi GeorgiaBuck,
No, you have nothing to worry about. You can't eat too much manganese found in its natural form (foods). It would only be potentially dangerous if you were taking supplements.
While I am not eating as much oatmeal these days, there was a time period where I was eating 3 cups (or more) of cooked oat groats every morning, on top of regular oatmeal in smoothies/desserts and sometimes as a snack itself! My manganese must have been through the roof.