Rivu is right ! There is no ready made solution for migration at the moment. You can hire a programmer to migrate the content . But I don't think it is a better Idea to move your MyBB based site to Oxwall
Forum section at Oxwall is just a module of Oxwall community software and it's not a full structured forum software .
Both the scripts vary in schema and several features (In the form of modules) you are using at MyBB may not be available with Oxwall forums module.