Ms Birtukan returned to Ethiopia earlier this month after seven years in exile in the US
Prime Minister Abiy Ahmed has carried out wide-ranging reforms since coming to office in April
The 42-year-old Abiy has also given half of the government's 20 ministerial posts to women
A former judge and leading opposition figure has been sworn in as the head of Ethiopia's electoral board.
Ms Birtukan Mideksa is the latest significant appointment of a woman to a key public office.
Ms Birtukan returned to Ethiopia earlier this month after seven years in exile in the US.
Key challenge
She was among dozens of opposition leaders jailed after the disputed elections of 2005 that led to the deaths of hundreds of people.
The BBC's Emmanuel Igunza in the capital Addis Ababa says she faces a key challenge in restoring faith in an electoral board that has constantly faced accusations of being manipulated by the state - and will oversee elections in May 2020.
Prime Minister Abiy Ahmed has carried out wide-ranging reforms since coming to office in April.
Underage marriage
These include making peace with neighbouring Eritrea after two decades of conflict, freeing political prisoners and welcoming back armed opposition groups that were once in exile.
The 42-year-old leader has also given half of the government's 20 ministerial posts to women.
Recently, he was commended for appointing renowned human rights lawyer Meaza Ashenafi - whose efforts to tackle the underage marriage of girls formed the basis of an Angelina Jolie-produced Hollywood film in 2014 - as the country's most senior judge.
