Occlusal guards may be soft or hard depending on their strength. Soft ones are meant for mild to moderate cases of bruxism while hard ones are meant for severe cases of bruxism.
Depending upon their method of fabrication, they may be stock trays, boil-and-bite or custom-made trays. While stock occlusal guards are made the one-to-fit-all measurement, boil-and-bite ones are more comfortable since they fit your teeth and gums better than stock occlusal guards. However, the best fit is got in custom-made night guards and mouth guards wherein your dentist takes specific measurements of your oral cavity and then gets your occlusal guard fabricated by a dental lab. They are the most comfortable and will start to feel natural in a few days of its wearing.