The agency won the business after a competitive pitch process and will be responsible for handling all TK Maxx's digital activity, encompassing online, social media and mobile work.
O&M has been tasked with creating digital advertising to amplify the brand's TV activity, which is expected to launch before the summer.
In the past, Golley Slater has worked on the retailer's advertising business, but it is now handled in-house.
TK Maxx has invested in a number of digital campaigns over the past couple of years, including social media activity to support its sponsorship of the Alexa Chung-fronted Channel 4 fashion show Frock Me.
The retailer also began engaging with mums online for the first time to promote its range of nursery products.
Mindshare handles TK Maxx's onand offline planning and buying account. The agency won the online business in 2009 from the incumbent, Arena Media.
TK Maxx's most recent ad campaign launched at Christmas and promoted its goods in-store and online, featuring the strapline: "Big Christmas, small prices."
Last year, TK Maxx made one of its "real-life" shoppers, Lauren, the star of its advertising campaign to introduce its "big labels, small prices" proposition.
O&M declined to comment on the appointment.