Thank you all so much for the supportive words. Yes, 
will definitely be in order later. DT2's offer is underwritten by his original college, yes - they will have him if nobody else wants him!
I did try to prepare DT1 for the possibility of DT2 getting an offer and him being redirected, and he was so good about it. He's been genuinely delighted to hear that one of their friends has got an offer, too.
Tbh though I thought it unlikely that it would be this way round. Both boys have virtually identical and excellent GCSE results and great predicted grades, but DT1 is way more passionate about his subject and more self-motivated. I thought he'd perform significantly better in interview as DT2 can be quite taciturn! Just goes to show - DT2 must have shown a different side to himself and/or done very well in the HAT.
I'm finding this really hard largely because of DT2's reaction to it all. A nonchalant shrug would be fine, or a bit of pleasure at having been given an offer, even if he intends to turn it down.
But there's so much angst and I can't unpick why. No one forced him to apply and no one's telling him he has to accept the offer. His brother has reassured him that he's fine about it (or is pretending to be!). I would be happier if he preferred one of his other choices and had positive reasons for it. But it all seems to be wound up with low confidence and some negative thoughts about Oxford.
The latter - that the work would be too hard or he wouldn't fit in - seem odd, as he knows I went to Oxford back in the dark ages, from a comprehensive school, and so can dispel these kind of myths. We've visited Oxford as a family - DH and I have friends there, though we live quite a long way away - and DT2 said a few minutes ago that he thought he might just want to go somewhere 'fresh and new': yet he wouldn't even consider Cambridge?!
It's difficult. His other choices include Durham (no offer yet) and Warwick (AAA, like Oxford). He has an unconditional offer from Newcastle which might be his insurance. He's never been to Durham or Warwick or done virtual open days, and hasn't seemed crazily enthusiastic about anywhere.
Yet he's dismissed any suggestions of years out, or considering other subjects. We only suggested the latter, back in the Summer, because he once said he wasn't passionate enough about History to read it at Oxford or Cambridge, when his teachers first suggested applying. We did point out that he'll need to be motivated and really quite interested to read History anywhere!
I don't really know how to help or advise him. Back in the day I just fell in love with Oxford at an Open day and didn't have any doubts. DT1 was the same, but I don't think he'll try again.
I feel that I'm spending all my emotional energy trying to support the son who did get an offer, while the one who didn't and who I think is quite gutted deep down, is just getting on with his work. It's topsy turvy!