Ms Horan, who has been acting president since January, had previously served as the organisation's vice president for marketing and membership.
"I am excited to have the opportunity to lead the OPA at a time in which the internet is taking on such an important role in our lives," said Ms Horan.
"I've been working in online publishing for years and have seen it go through a dramatic evolution.
"With the maturing of the medium, there now exist incredible opportunities for consumers, publishers and advertisers.
"I look forward to continuing and expanding the OPA's vital role in addressing the opportunities and challenges that lie ahead."
Ms Horan joined the OPA from Zinio Systems where she was the vice president of marketing.
"For nearly two years Pam has been a vital part of the OPA, leading our marketing efforts and serving as interim president," said Debora Wilson, chairman of the OPA.
"It became increasingly clear during our search for a president that Pam was exactly the right person to direct the OPA going forward.
"She has a passion for, and a great deal of experience in, the internet sector and has done a terrific job leading and growing the OPA and its membership."
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