It's same as ohhhhhh Oh+hhhhhh=오+~ We usually use this mark '~' to express the vibration of voice or extension(?) of speaking. Actally we normally call this mark "물결표시" which means "mark of the wave" and we consider that the wave and vibrating look and sound similar.

Oh ~ you are good at Korean a little more. "~" it doesn't have such a huge meaning.

오 ~ 한국말 좀 늘었네 And what does "~" mean? ~ is just a filler to make the message feel more casual/positive, similar to a smiley. Korean = 한국말 좀 = 조금 = a little 늘다 = improve (늘었다 improved) -네 to be surprised about the fact. 오 ~ 한국말 좀 늘었네 Oh your Korean has improved a little! ~