Welcome, SIP.
First - I applaud you for wanting to help. But keep in mind, you are not responsible for your brother, or for your mother. As long as you are helping,that's OK< but don't let any guilt creep in. It is very unhealthy for you now and in the future.
Second - ODD is unlikely to be the main problem. It is highly likely there is an underlying disorder (such as ADHD, or any one of a number of possibilities) which need to be dealt with. Once they are being managed, then a lot of the OFD issues also are dealt with.
Third - I am no help to you in finding a therapist. I'm not even in the same hemisphere as you. I'm not sure if it's even possible on this site to find you a therapist. But there will be ideas on how to find one. In the meantime, the best therapy is often from the home environment, so PLEASE read a book that many of us here have found to be useful - The Explosive Child by Ross Greene. Get it from the library, or Google it online. Trust me, there is stuff in this book that works. Get your mother to read it too, and your step-dad. Or you read it, and then explain it to them. While you wait to get him seen by someone, this can change lives. Therapists are useful, but not the magic answer. There is always more needing to be done at home. That book is like a handbook, a guide book on how to change YOUR behaviour toward your child, in order to help the child change for the better. It is not a cure, but it can help you find your way out of the darkness at least to where it's light enough to see the path.
Keep in touch, let us know how it's going. There will be more along soon.
Marg