O'Brien (5-11, 176), a right-shot center from Thayer Academy (Massachusetts), was the first high school player chosen in the first round since the Florida Panthers selected Nick Bjugstad of Blaine High School (Minnesota) at No. 19 in the 2010 NHL Draft.
"He's a really smart player, really good hockey sense," Hextall said. "He's competitive, strong, and he's got a little agitator in him. He's got a good shot and sees the ice well. He's a little bit undersized, but we think he's going to be a fit kid and a strong kid. We really liked him."
Six Sweden-born players were chosen in the first round to equal a record set three other times (1993, 2009, 2011).
Additionally, 14 defensemen were chosen in the first round, breaking the record of 13 from the 2012 NHL Draft.
Rounds 2-7 are Saturday (11 a.m. ET; NHLN, SN, TVAS).
There were only two trades made during the first round: The Rangers sent the Nos. 26 (defenseman Jacob Bernard-Docker) and 48 picks to the Senators for No. 22 (defenseman K'Andre Miller), and the St. Louis Blues traded the Nos. 29 (defenseman Rasmus Sandin) and 76 picks to the Toronto Maple Leafs for No. 25 (right wing Dominik Bokk).
"The only thing I'm going to say is there was what, two trades made today? It was a very quiet day on the phone," Senators GM Pierre Dorion said. "I made a lot of calls and many calls, and the GM said, 'Hey, you're my first call today.' And that happened in the afternoon."
The last time the top of the draft was dominated by European players was 1999, when the Atlanta Thrashers chose Czech Republic native Patrik Stefan No. 1 and the Vancouver Canucks selected Sweden-born forwards Daniel Sedin and Henrik Sedin with the second and third picks. Then at No. 4, the New York Rangers chose Czech Republic-born forward Pavel Brendl.