Would 1:72 scale military vehicles look okay on a OO gauge layout?

Whether or not to use slightly off-scale models on a layout is a subject that's traditionally both subjective and divisive. Modellers of a purist nature would rarely tolerate such a crime while the owners of layouts intended to casually entertain the family will generally adopt an 'anything goes if it's fun' view on the subject. Most modellers will fall somewhere between the two.

With that in mind, whether or not to use 1:72 scale models of any kind on a OO gauge model railway can only be your decision. The main consideration for anyone planning to mix scales is whether or not you have inadvertently done it already. Many models (other than rolling stock) used on OO gauge layouts are in fact HO gauge. That is particularly true of many figures, buildings and road vehicles and although 1:72 scale is not too far removed from the 1:76 scale of OO gauge, it is quite a jump from the 1:87 scale of HO.

If you're still not sure what to do or you're completely baffled by the jargon, maybe this will help:

In 1:72 Scale, 1 ft on a prototype is represented by 4.23 mm on the model.

In 1:76 Scale (OO gauge), 1 ft on a prototype is represented by 4 mm on the model.

In 1:87 Scale (HO gauge), 1 ft on a prototype is represented by 3.5 mm on a model.

That means if you take two models of the same real-life 20 ft long vehicle, but one of the models is in 1:72 scale and the other is in HO scale, the difference in length between the two models would be 14.6 mm. That may sound like a totally acceptable difference, but don't forget to factor in the expansion in height and width too before making your decision.