Complex sentence: O16.5 in the United Kingdom is a model railway scale/gauge combination of 7mm to the foot, the same as British O scale (1:43.5 ratio) with 16.5mm gauge track, which is used by HO/OO gauge model railways.
Split into simpler sentences: O16.5 in the United Kingdom is a model railway scale/gauge combination of 7mm to the foot. This is the same scale as British O scale (1:43.5 ratio) running on 16.5mm gauge track, which is used by HO/OO gauge model railways.