Can an oral-b moldable night guard from amazon.com be enough to treat tmj? or should it be one done by dentist?

Custom splint: Dentists who specialize in TMJ interventions provide custom made splints which are fitted to each individual. Amazon has generic splints which may or may not properly align mouth and jaw. In other words, this is a question of quality and specificity vrs. assemblyline low cost factors. Hopefully, your dentist could better explain the differences.

They can help.: Over the counter oral appliances can be appropriate for short term use. The question is...what is the cause of the problem ? That answer will have to be given by a trained, knowledgeable dentist.