O'Reilly is leaving 12 months into a three-year deal with the BBC to explore other opportunities and set up a support network for women facing discrimination in the workplace.
It is just over a year since O'Reilly successfully sued the corporation for age discrimination.
An employment tribunal ruled that she had been dropped from Countryfile because of her age and victimised by management, who blamed her for newspaper stories criticising the BBC for dropping middle-aged women presenters.
O'Reilly has since returned to the corporation as co-host of BBC1's daytime Crimewatch spin-off, Crimewatch Roadshow, but did not make any more appearances on Countryfile.
She was one of four female presenters, all in their 40s and 50s, who were dropped from the show in 2009 when it was moved to a new Sunday peak slot. It has subsequently become one of the BBC's most popular Sunday night shows.
O'Reilly said: "I have had a rewarding time at the BBC over the last year but am now choosing to move on and work on other projects for a while including my charity, Women's Equality Network."
A website offering help and support to women facing discrimination in the workplace, O'Reilly is setting up the Women's Equality Network with the lawyer who secured her tribunal victory, Camilla Palmer. It will launch next month.
"We realised from the amount of emails and letters Camilla and I were getting how many women didn't know where to go for help and advice about all kinds of discrimination – sexism, racism, ageism. It really did surprise us," O'Reilly said.
"The whole aim of the network is to help women not [have] to resort to legal action. With a bit of advice and a supportive voice at the other end of the phone, women may be able to stay in their job and work the problem through."
O'Reilly said she thought broadcasters' attitude had begun to change towards employing older women, but said it was a long-term process.
BBC Trust chairman Lord Patten said in September last year that hewanted to see a stronger voice for older women on television.
BBC1 controller Danny Cohen has also said he wants to do more for older viewers and "reflect the audience back to itself". The average age of BBC1's audience is around 50.
Not everyone has been so sympathetic, however. Former Crimewatch presenter Nick Ross told an Edinburgh TV festival audience last yearduring a session on ageism in the industry: "Like it or not, TV is a lookism medium. In 40 years of TV I have never worked with a minger."
O'Reilly said she had discussed the issue a number of times with BBC director general Mark Thompson.
"He really wants to make sure there is a fair representation of older women," O'Reilly said. "I have always said he is a man of integrity and have found him to be so.
"There are changes – they have come slowly – but you can't bring change overnight, no matter what it is in television because of long-term commissions and so on."
O'Reilly said she wanted to continue to work as a broadcaster and journalist and has been lined up for a return to the BBC with a radio project in the autumn.
She previously presented BBC Radio 4's environmental programme Costing The Earth as well as Women's Hour and Farming Today.
A BBC spokesman said: "We would like to thank Miriam for her work on BBC television and radio and we wish her well in her new challenges but hope that there will be opportunities to work together again in the future."
Article from The Guardian