I don't know if this is going to be of any use at all, but these are the rules we use at IPMS to define exactly what is and isn't OOB.
Out-of-Box (all scales, all subjects)
This category is designed to produce a model within the limitations of the following conditions:-
i) The model must be built using parts contained within a single kit as per the instructions.
ii) No modifications may be made to those parts (no gun barrels or exhausts being drilled out etc)
iii) The model must be built in the colour scheme, markings and decals supplied in the kit.
iv) Filler or model putty may be used for filling joints and gaps only.
v) Sprue or other material may only be used if specified in the instructions.
Cheers