O???Brien, 31, will join up with his brother Niall at Grace Road from May 15 to June 26 and overseas signing Grant Elliott, who was so impressive for New Zealand in the World Cup.
He has played in 125 T20 games in his career and needs just 20 runs to reach the 2,000 mark, with his strike rate an impressive 132.35.
O???Brien has had spells in England with Gloucestershire, Nottinghamshire, Somerset and Surrey and has played in the Bangladesh Premier League and Caribbean Premier League.
The club???s chief executive Wasim Khan said: ???It is great to have Kevin as part of our NatWest T20 Blast team in 2015.
???Kevin adds further international experience to a line-up that includes his brother Niall, Grant Elliott, Mark Cosgrove, Clint McKay and Rob Taylor, and we look forward to seeing him in action at Grace Road.???