Both dental inlays and onlays are indirect restorations created in the dental lab. The difference is that inlays fit within the grooves of the teeth and do not include the tooth cusps or peaks. Whereas onlays are meant to cover the tooth cusps and peaks.
Onlay: Earlier, dental onlay restorations were only cast with gold. But nowadays, more and more people desire tooth-colored onlays. Therefore, onlays are now available in composite, resin, or ceramic/porcelain material. These tooth-colored materials can be fixed to the tooth with strong bonding cement, improving its retention and durability.
Inlay: Like dental onlays, inlays are also made of solid pieces of gold, resin, composite or resin material
Both dental fillings and onlays are excellent treatment options for decayed, broken, or chipped teeth. There’s no way to declare one better than the other. The requirements differ slightly. Fillings are great for smaller cavities, and there is sufficient tooth structure remaining. On the other hand, onlays are great for large cavities with a considerable amount of decay and not enough tooth structure left for a filling.