おなか is very common word to say "stomach" or "belly," but is not a name of an organ. For Example; おなかが　すいた。　I'm hungry. おなかが　いたい。　I have a stomachache. (NOT pointing a specific organ.) In contrast, 胃 is a word to point specifically "stomach" as an organ, like 胃がいたい。 So I would say おなか is more common :)