Ms O'Brien said her comments were made "off the cuff" but admitted her "use of language was unacceptable".
She is still set to run in the General Election on 12 December.
Ms O'Brien, a former commission officer with RAF Air Cadets, was selected to be the Conservative party candidate for Gower in October after an open primary in which residents were able to participate as opposed to just party members.
Having lost Gower to Labour in the 2017 election, the seat is a top Tory target in Wales.
In the posts following the broadcast of the first episode of Benefits Street five years ago, Ms O'Brien said: "Benefit Street..anyone else watching this?? Wow, these people are unreal!!!"
Responding to another user's comment, she said: "My blood is boiling, these people need putting down."
In a statement released on Sunday, Ms O'Brien said: "These comments were made off the cuff, a number of years ago.
"However, I accept that my use of language was unacceptable and I would like to apologise for any upset I have caused."
Benefits Street, which featured the residents of James Turner Street in Birmingham, was a ratings hit for Channel 4 when it was broadcast.
But a total of 887 viewers complained to the broadcasting regulator, claiming the show had vilified and misrepresented benefits claimants.
Ofcom decided the programme did not breach broadcasting rules.
Ms O'Brien is due to stand against Labour's Tonia Antoniazzi and Sam Bennett of the Liberal Democrats. Other parties are yet to confirm their candidates.
I have often found in the hustings for Pontypridd that the Tory candidates often portray themselves as moderates , who even say they disagree with some Tory policies.
Of course they never win and we never really know whether they are moderate or not.
Despite the lurch of the Tories to even more right wing policies many of the candidates who are challenging other parties of the centre and left will not be adopting the rhetoric of the Johnson,Rees Mogg and the European Reform Group who seem to have seized control of the party.
If you don't like the Non-Tory party in a area where it is a clear two horse race between them and the Tories (like the Gower) , it probably best not to risk the likes of Francesca O'Brien winning, even if she suddenly seems to be portraying herself as a moderate.