Whisk together soy sauce, brown sugar, water, garlic, onion, ginger, black pepper, oregano, red chilli flakes, cayenne pepper and paprika in a large glass dish or ceramic bowl. Add chicken thighs and toss to coat evenly. Cover the bowl and marinate the chicken in the fridge for at least 1 hour.
Preheat an outdoor BBQ for medium heat and lightly oil the grate.
Remove chicken thighs from the marinade; discard any remaining marinade. Barbecue chicken thighs on the preheated BBQ until cooked through, about 15 minutes per side.