I've been to HBOT thrice. First time was like 8 weeks from onset or something, didn't really do much, but it was a mental relief at least. Second time was 3 weeks from re-exposure onset, helped significantly, made the tinnitus in my right ear disappear completely. Third time was around the same time from onset, and was also effective, nearly silencing my tinnitus in general. It also positively affected my older tinnitus that I had gotten 3 months prior. Each time took only 5 sessions, one per day.
I would definitely recommend this treatment if you can get it within a month or so from your onset.
Also you need to give the treatment time to affect. It's not often immediate, but instead the recovery takes place within a couple weeks after the treatment. It merely gives your body the ability to recover on its own, and it takes time for it to heal all wounds. Don't expect results in one or two days. And yes, it's completely normal for your tinnitus to spike a bit for a few days or weeks after treatment, and it's a sign that it actually did something. It happened to me every time and went away on its own, so I was only left with positive results.