When i use my occlusion splint or night guard i keep mouth breathing whenever i use its becoming annoying can someone tell what to do to solve this?

Your night guard may need to have some adjustments. Do you snore? Have you been evaluated for sleep apnea

With every medicine there are side effects. In the c are of an occlusal orthotic mouth breathing is a common side effect. You might want to try wearing it 8hrs/day instead of at night when you sleep. Your Dentist may be able to redesign the appliance to allow better sleep.