
I don't know about OES particularly, but I do know that £3500 is a very high price for a 5 month pup. I can only assume that it was a potential show dog that the breeder has run on and it hasn't developed as expected, and is being sold as a show pup but not a good fit for them. If it's being sold as a pet then it's very expensive; I know it's cost money to rear but older pups are usually no more costly than the usual 8 week pups because they are, let's face it, less 'desirable' having lost the 'cute puppy' factor and, unless the breeder has been very conscientious in training and socialisation, potentially has bad habits that, at that age, can be very hard to overcome. You need to ask lots and lots of questions about how it's been reared - in the house so it's fully house-trained? Is it used to going into towns and is used to walking in crowds and traffic? If it hasn't been off the breeder's premises much then it's at a big disadvantage and fear of these unknown experiences can be a permanent part of its nature.
I assume the younger pup is kennel club registered, and from health tested parents? Regularly wormed, microchipped and possibly had its first vaccination?