Stack Exchange network consists of 183 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers.
It is doubtless onomatopoeia, and shows up in a version of the story about a woman ghost who buys candy for a child each night. What does it represent?
オギャー or オンギャー is a baby's crying sound, especially a sound at the moment when a baby was born. We are relieved when we hear the sound at the moment because the baby was born healthy.
オギャー is used more than オンギャー.
Here in here you can find the equivalent sound in English as "waa or waaah".