The 28-year-old becomes the club’s fifth signing of the window – following Hayden Carter, Tyler Walker, Ollie Webber and Denver Hume through the Fratton Park doors.
The former Millwall ace has signed a permanent deal until the end of the season.
The departures of John Marquis, Ellis Harrison and Gassan Ahadme had left Danny Cowley’s side relatively light in the striking department, despite Walker’s loan arrival from Coventry.
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But the acquisition of the Republic of Ireland international has bolstered the Blues’ squad for a potential push towards the top-six over the second half of the season, with the forward leaving the Stadium of Light after scoring 12 goals in 65 appearances in red and white.
O’Brien’s arrival looks likely to end the club’s transfer activity this month, with potential moves for Jamie Lindsay and Lewis Wing failing to materialise.
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