On the post-it style sticky note, Oxfam will encourage people to ask for its white wristband to wear for World Poverty Day on 17 October.
The second brand to use a MemoStick will be broadband telephone provider Vonage, with a campaign in the Observer later October.
The post-it style advertising, secured to the front page of each title, is available on all of The Observer's monthly magazine titles, which also include Music, Food and Woman.
Michael Court, director of All Response Media, who negotiated the deal, said: "This is a wonderful, dynamic idea that as a media first is testament to how innovative The Observer has become. The format will be a fantastic mainstay for direct marketing and brand advertisers alike."