Yes has always had a high rate of keyboardist turnover. Tony Kaye lasted just over 3 years initially. His replacement was of course Oliver's dad, but Rick lasted less time, leaving after 2 years and 8 months. Next came Patrick Moraz for an even shorter spell: 2 years, 1 month.
Rick was back, but only for just over 3 years, before the shortest spell of the keyboardists we usually count, Geoff Downes, at a bit under a year.
Tony Kaye then returned... or rather joined Cinema, which became Yes. He seems to have been in the band for around a year before leaving, being replaced by Eddie Jobson for a few months, and then returning. Kaye was then continuously in Yes for by far the longest period of any keyboardist, another 11 years.
Counting ABWH as well, Rick had his longest spell in Yes of around 4 years from the beginnings of ABWH to the end of the Union tour. Given there were plans for him to be involved in Talk, maybe we should count that as longer. After Kaye's departure, Rick again returned, but this time quite briefly, about 1.5 years. Igor Khoroshev lasted about 3 years. Rick's latest stint in the band was for 2 years, 2 months.
Compared to these previous Yes keyboardists, Oliver has done pretty well. He's been in the band about three and a third years, although the cancellation of the 2008 tour meant his earlier months weren't very busy. (Even if you count only from the beginning of rehearsals for the In the Present tour, Oliver is still on 2 years, 5 months.)
Oliver's period in the band is longer than all but one of his father's; longer than Kaye's first time round; longer than Downes, Moraz, Khoroshev or Jobson. Any Yes keyboardist who gets to 3 years should probably be looking over their shoulder(!); only Tony Kaye has done substantially better.
What marks Oliver out, however, is the possibility that he won't appear on any studio material. While we now know Downes is the main keyboardist on Fly from Here, it's unclear whether any of Oliver's work will be used.
Switching keyboardists while making an album is familiar territory for Yes. Rick first came in when the band were working towards Fragile
If Oliver isn't on Fly from Here, however, the only precedent for someone touring as a member of the band but not seeing a studio release is Tony O'Reilly's few months as drummer in 1968.
The other lessons here are that Downes perhaps deserves another go at the job more than most ex-keyboardists, given how brief his first time was, and most Yes keyboardists get to come back at some point! In the mean time, here's good luck to Oliver for his career after Yes.