The 53-year-old broadcaster is suing the BBC for sex and age discrimination after losing her job on the programme when it transferred to a primetime slot on BBC One, reports PA.
In a witness statement handed to an industrial tribunal yesterday, O'Reilly said that Countryfile director Dean Jones "sent a shiver down my spine" when he warned her that HD could be "crunch time" for her BBC career when it was introduced in February 2008.
She said: "I do not believe that a man would be asked about his wrinkles nor offered hair dye. It was clear to me that this was a reflection of the BBC's view that women on TV needed to look young."
O'Reilly, who has worked at the BBC for around 25 years, was informed in November 2008 that she would no longer appear on the rural affairs programme.
She said that she was "devastated" by the news that she would lose her job when the show relaunched in April 2009 with Julia Bradbury, then 38, and Katie Knapman, then 36.
Michaela Strachan (42), Juliet Morris (52) and Charlotte Smith (44) were also taken off the show, but main presenter John Craven, 68, and Adam Henson, in his 40s, were kept on.
In her statement O'Reilly said that she was not told why she would no longer have a role on the programme, only that Countryfile was being "refreshed".
"This news was a huge disappointment," she added.
"I was of course happy for the programme and its success but felt that this success was linked to the presenters' connection with the audience."
O'Reilly also claimed that she was victimised by the BBC because she was suspected of having "leaked stories about the BBC and ageism". The hearing continues.