The 21-year-old's arrival will offset the loss of Maximilian Beister, who was ruled out for the rest of the season after rupturing cruciate knee ligaments in a friendly against Vitesse last week.
Reunion for van Marwijk
HSV head coach Bert van Marwijk is familiar with John's abilities after having given him his international debut for the Netherlands in February 2012. "I know Ola well and I'm confident he'll make us stronger," said the 61-year-old. Sporting director Oliver Kreuzer also added that the injury to Beister had been a factor in the deal, adding, "In our present situation it was vital that we acted [in the transfer market]".
Born in Liberia, John moved to the Netherlands as a child, joining FC Twente in 2002. After breaking into the first team, he scored nine goals and provided 16 assists in 46 league games, also amassing UEFA Champions League and Europa League experience, before transferring to Benfica in the summer of 2012.
John, whose two older brothers Paddy and Collins are also professional footballers, made 24 league appearances for the Lisbon outfit in his first season and helped Benfica reach the final of the 2012/13 Europa League, which they lost to Chelsea FC.