Mr Edgar, who has vigorously opposed the introduction of voluntary student membership, said he would like to apologise to Sir Roger for the comments, which, he said, he posted "in the heat of the moment" near the end of a parliamentary debate on Act's Education (Freedom of Association) Amendment Bill on Wednesday.
The post, left at 8.42pm, said: "Get f... you dinosaur ... just trying to give yourself a legacy because you know you're getting too old. You should actually debate the Bill with Pete [Hodgson] or Grant [Robertson] ... you'd get torn to shreds. [expletive]."
Mr Edgar said the comment was on the page for about a minute before he took it down.
However, a copy was sent to the page's administrator and the comment appeared on the Kiwiblog site last night.
Mr Edgar said last night he knew his behaviour was not acceptable from someone in his position. "It was not my finest hour. But at the end of the day, I am a student, and we make mistakes. Everyone makes mistakes."
He would "definitely" apologise to Sir Roger, in either another post on his Facebook page or an email.
He said he had informed the OUSA about the incident, and it was not taking any action.
Although Act and Kiwiblog last night called his leadership into question, he said he did not think Otago students would listen.
Sir Roger was unavailable last night but the Bill's sponsor, Act MP Heather Roy, said she found the comment "pretty appalling" from someone in such a position of authority.
Mr Edgar posted a comment on her Facebook page too, but it was not obscene.
Mr Edgar needed to differentiate between his persona and what he said as OUSA president, she said.
"I would have thought it would be prudent of the president of the OUSA to be careful about what he says."
He was in charge of a multimillion-dollar business, which required transparency from its leader, and represented the largest student body in New Zealand.
"He should be taking those responsibilities very seriously. I have to question whether Logan is up to the job."
