Josh
O gauge, in the UK is 7mm = 1 Foot, or 1/43 scale, track gauge is 32mm. G Scale, is not a true 'scale' as there are many varying scales running on 45mm gauge track. If you take LGB products as being the 'norm', they are nominally (given their rubber ruler practice) 13.5mm = 1 Foot, or 1/22.5 scale. You should also remember that G scale is the narrow gauge brother of Gauge 3, full gauge, 1/22.5 scale, which uses 64mm gauge track. So the short answer is yes, your O gauge will look too small, unless you are using your models to the rear of the G scale, to achieve the illusion of depth perspective.
David