The appointment, after a three-way pitch, is the precursor to an £8 million national TV, radio and press campaign, which will break on Tuesday.
MWO has been looking to take on another ISP since losing AOL, for which it created the famous Connie character. AOL switched its £29 million UK business into Grey London in October 2003.
MWO's brief is to promote the launch of Tiscali's new broadband offering, which, Tiscali claims, will provide stiff price competition to its most significant competitors, AOL, BT and Wanadoo.
Two 30-second spots will mark the first time the ISP has used UK-produced TV work.
The offer is being underscored with a strapline: "With Tiscali, everyone in the land can afford broadband." Media planning and buying remains with OMD UK.
Tiscali has grown substantially since its formation in 1998 as a local telecoms company in Sardinia.
Taking advantage of the deregulation of the Italian telecoms market, the company spread its services across the country before pursuing an aggressive acquisition programme throughout Europe.
It acquired a UK presence three years ago via the acquisition of a number of ISP brands including Currant Bun, World Online and Liberty Surf.
The new campaign is intended to up the stakes in an increasingly competitive UK broadband market that, so far, has seen little price differentiation between rival offers.
The commercials were directed by Steve Bendelack, who is best known for his work on the TV comedy series Little Britain and The League of Gentlemen.