The 27-year-old has spent the previous three years in Sweden where he has featured for Piteå IF and Vasalunds IF.
He has appeared for the Silkmen during their pre-season preparations and will now ply his trade for Mark Duffy's side.
Born in Macclesfield, O'Keefe came through the ranks at his hometown club in 2014, before departing the country to seek opportunities abroad.
In 2017, he joined third-division side CF Fuenlabrada, and was in the squad when they faced Real Madrid in the Copa Del Rey.
O'Keefe returned to the UK in 2018 and enjoyed a one-year spell at Loughborough University before travelling to Gibraltar to link up with Europa Point FC and Bruno's Magpies.
Photo credit: Piteå IF
Keep up-to-date with our exclusive email newsletters.
Subscribe