Due to how streaming is implemented in general, there is always some delay between you feeding video and audio into OBS and your viewer getting video and audio out of his browser. This delay is always some seconds.
If you want to speak with one of your viewers directly without this delay, you have to establish an additional direct voice channel to him, so he can listen to what you say directly, not via stream. If you use some voip gamechat, you have your direct channel in addition to your stream. Make sure your mic is going to that voicechat in addition to going into OBS, and make sure your viewer is listening to that channel and not (only) to your stream. If you want your viewer's voice appear on the stream, you have to grab the voicechat as additional audio source and feed it into OBS.