Doyle, 21, has joined from Morecambe and O'Brien, 24, from Bury.
Centre-back O'Brien spent last season on loan at Halifax, while Doyle had loan spells at Chorley in 2014 and 2015, making 59 appearances.
O'Brien is is eligible for Saturday's league opener against Dagenham & Redbridge but Doyle is sidelined for up to four weeks with an injury.
Find all the latest football transfers on our dedicated page.
National League side Southport have added to their squad by signing defenders Keil O'Brien and Chris Doyle.