Yep, you need a compressor. It'll make quiet speech louder, and limit louder volumes so you don't kill headphone users.
I ended up grabbing a mixing board (Behringer 1204 USB) that included it (many Behringer boards have this functionality built in, and it works fairly well... I believe the cheapest is the 502?) after I literally blew out two viewers' headsets who had stuff turned up too loud. :b
Too many streamers don't provide a good (normalized across the full range) volume level... quite a few send VERY quiet audio, causing people to turn their volume WAY up, far past where it should be if they were sending at proper levels. This is also why a lot of people complain about ads BLASTING them with volume. The ads are set to the proper volume level, and are extremely loud, as compared to the streamer who is sending super-quiet audio.
You should be shooting for about 3/4 on the OBS volume meter during normal speech, with the game at around 1/3-1/2 to be heard clearly (due to no audio ducking/attentuation-on-speech functionality being available in OBS). If you're yelling, pegging the meter is fine if you have a compressor... but you don't want to be when just talking. Likewise, you don't want to be hovering around 50% while talking or you won't be sending the appropriate levels.
