Hello, I'm Dr. Kara. I have over 20 years of experience as a veterinarian and I'd like to help with your concerns about Otto's unpredictable aggression.
I can understand how heartbroken you must be about his behavior, especially because at home with your family he is a gentle lamb.
I think that your concerns about him are absolutely valid in that his behavior is so unpredictable and he may one day cause serious harm to someone that cannot be undone. In many ways it would be better if he were aggressive with a particular type of person (male, tall etc.) because then we would know how to work with him. Because his behavior is unpredictable training him will be very difficult and he will never be a dog that you can really trust. You may also be legally responsible for this behavior given his track record of previous bites. If you choose to move forward with this and want to keep him you must be ready to work very, very hard and be on guard with him his entire life.
Every time he leaves your home now he must wear a muzzle.
In fact ideally until you gain better control of him he would not leave your property. I realize that may not be possible, thus my recommendation for the muzzle.
I see that you have tried some obedience training with him and did not have much success. I understand it is difficult to control a large dog, but that is all the more reason to get him to learn and flawlessly follow basic obedience commands. If he is looking to you and sees you as the boss in the relationship then he should listen to you and thus it would follow that unless you told him to bite (which you won't ever) he won't do so.
You need to get a veterinary behaviorist to work with you by observing Otto's behavior with you in the home and out on a walk to look for aggression triggers with him. It may be hard for you to see these as you are not an impartial observer trained to look for subtle clues that dogs may give us. By identifying triggers, and redirecting him before he begins to get aggressive you can avoid bites. If you don't have a behaviorist then getting an experienced trainer to work with you is next best.
I also recommend that you make sure that Otto knows where he is as far as leadership roles. You are the boss and needs to look to you and not make his own decisions on how to act. If he is currently allowed on furniture he should no longer be for the immediate future. People get on furniture and by allowing him up you are putting him on your level, which he should not be.
You should be practicing obedience commands with him daily.
I also recommend using a program called nothing in life is free to help him realize that you are in charge and give him all that is good.
Please see this link for more information: http://k9deb.com/nilif.htm
You have a huge task ahead of you and it may not work, but if you want to keep him then you need to work as hard as you can.
Please let me know if you have further questions.