No, Ogdon certainly didn't record OC by sight-reading it. For a start, he played it to the composer and the work's dedicatee (Hugh McDiarmid) at Ronald Stevenson's house some time in the 1960s. And I know he did at least some practice before the recording because I was there and he told me so (and I choose to believe him because just about everyone who knew him will confirm John was apparently incapable of telling any kind of lie).
However, there's one thing I witnessed at the recording session of that piece that gives some idea of his sight-reading. OC is mostly written on 3 staves rather than the usual two, the division between the hands being entirely up to the player. They may be treble-treble-bass, treble-bass-bass, whatever. At one point, the producer (our good friend Alistair Hinton) realised that one of the score's many misprints results in the middle stave of the three having the wrong clef on it. John had played it as printed. Now personally I would say that having to re-think a passage like that is possibly even harder than sight-reading it, and the passage in question (don't ask me which page, this was nearly 25 years ago!) was of course rock-hard in the first place. John corrected it immediately, basically note-perfect, with no semblance of mental effort.
I reckon I once caught John sight-reading in a public recital. He player Grainger's paraphrase on Tchaikovsky's Flower Waltz, in which there is a very obvious misprint - one bar is missing. Because the theme is so well known, every pianist will do what I did when sight-reading this piece - stop at the mis-print, take a pencil and make some mark to indicate what should be there, and always play it correctly in future. John, in front of several hundred people, played the misprint as printed. In fairness, it probably wasn't his greatest performance of anything ever, but it wasn't half bad either.
The thing is that freak talents like that are perfectly capable of learning a truly new concerto in a couple of days. There are several stories about Busoni, for instance, learning new pieces just from reading them and then giving a performance - not just a playthrough but a real musical performance. It's just like the mathematical freaks who can multiply two 30-digit numbers instantaneously in their head. The rest of the world takes weeks or months. A relative of my wife's is married to a prominent concert pianist who is a merely 'good' (i.e. not freak) sight-reader, who can learn a concerto in a week if he has nothing else to do but prefers to spend a couple of months at least. That's more like the norm, frankly. That pianist in turn is very friendly with Radu Lupu, who I understand takes many months to learn a concerto - hence his repertoire is quite select. But he's one of the finest and most-sought-after pianists on the planet. It's the end result that matters!