O'Reilly took the BBC to an employment tribunal for alleged age and sex discrimination, after she and four other female presenters were ditched from Countryfile when the show moved to primetime Sunday evenings.
The 53-year-old claimed that she was warned "to be careful with those wrinkles when high definition comes in" just months before she was dropped from the programme.
Giving evidence to the tribunal in November, former BBC One controller Jay Hunt hit back against the accusations of ageism on Countryfile, claiming that she would never discriminate against women over 40 because she is one herself.
However, the tribunal has today upheld O'Reilly's claims of age discrimination and victimisation, but dismissed the charges of sex discrimination. The presenter will now receive significant damages from the BBC.
Speaking to The Guardian, O'Reilly said: "Words cannot describe how happy I feel. It's historic and it's going to have huge implications for all broadcasters."
Last week, it emerged that the BBC spent more than £600,000 dealing with industrial tribunal cases in 2010, including almost £400,000 in payouts to disgruntled former employees.