One who is apt to disclose too much personal information, typically that which is related to their personal life and which would makes the recipient uncomfortable or which the recipient would prefer not to know. I knew Jeremy was an oversharer, but I didn't expect him to tell the boss what the itch cream was for. Oh yes, Janet is an oversharer. Like, who cares that her fourth hamster died—she's just going to go out and get another one anyway!