To look at an oyster, even an open one, it would be difficult to determine its gender. Oysters, like many invertebrates are hermaphroditic. Most specifically they are sequential hermaphrodites. This means they have the ability to be both sexes although not at the same time. Oysters are protantrous hermaphrodites, meaning that htey start off life as male, then most will change to female.
Oysters reach puberty and have the ability to reproduce after about one year.
After that point, almost all of them are male, but they begin to change to female at this time. Although other species of oysters can change back and forth between male and female, Crassostrea virginica cannot. Therefore, one can make an educated guess as to the gender of an oyster depending on size, but it is important to realize that you may not be correct in your guess. Larger oysters are probably females while the small ones are males. Maybe it has something to do with more energy is needed to produce the larger eggs than sperm.