The OFT concluded that Handpicked Media was in breach of Consumer Protection from Unfair Trading Regulations 2008, following a six-month investigation.
Handpicked had used people to comment and blog on behalf of its clients using media including Twitter.
A statement from the OFT said it took the view that when comments are made without "sufficient disclosures" to say they have been paid for, it would be "deceptive".
Handpicked cooperated with the OFT and has now agreed to "clearly identify" when comments have been paid for.
Handpicked director Krista Madden said: "Our blogs and sites are instructed to show full disclosure on their blogs if it is paid for content. Twitter has been a recent addition to social media and no rules were in place at that time.
"There were no clear rules on commercial activity for blogging in this country and we were very happy to work with the OFT to make sure that there is total transparency moving forward with set rules so that consumers can make informed decisions. We do hope this means that all those taking part in all types of social media activity will follow our example now in the UK."
Heather Clayton, senior director of OFT's consumer group, said: "The internet plays a key role in how people purchase products and services and the importance of online advertising continues to grow. The OFT has bolstered its expertise in this area and is taking targeted action to ensure that the law is clear, increase business compliance and empower consumers.
"The integrity of information published online is crucial, so that people can make informed decisions on how to spend their money. We expect online advertising and marketing campaigns to be transparent, so consumers can clearly tell when blogs, posts and microblogs have been published in return for payment or payment in kind. We expect this to include promotions for products and services as well as editorial content."
An OFT spokesman said the action "will provide clarity and certainty to companies, their marketing departments and advertising agencies on the disclosure requirements that apply in this area, consistent with the Consumer Protection Regulations".
The OFT recently warned online retailers to maintain the same consumer protection standards as those on the high street, and warned all retailers or outlets against misleading pricing.