The UK deal, called MMS Interconnect, means that O2 customers are able to send and receive multimedia messages - featuring pictures, sound and text - to Orange and T-Mobile subscribers, as well as other O2 subscribers.
According to mmO2, a full interconnection between network operators will help drive the widespread adoption and use of MMS-enabled and camera phones in the UK.
"Our goal is to make MMS as easy to use as SMS," said Kent Thexton, chief marketing and data officer at mmO2.
"We will continue to work with industry players such as the Mobile Data Association and the GSM Association in order to grow the range of MMS-based services available both in the UK and globally."
If a customer of an operator that is not part of the UK interconnect agreement is sent an MMS message by a customer of an operator that is, they will receive a text message asking them to visit a given web site address to get the full message.
In Germany, O2 customers have been able to interconnect with T-Mobile and Vodafone customers since December 2002.
Analysis, p18.