Currently an SDK (Software Development Kit) is not available for OFP and there's no promise that one will be available in the future. In an August 29th interview with BIS's Marek Spanel, OFP's project leader, Marek stated:
"We are working on the mission editing documents right now and the first version of them should be online within the next few weeks. There’s no final plans about the SDK. We will consider what tools to reveal to the community and how to best to it. Most likely, even the first tools will not be released to public in the next weeks. We still believe there’s a big potential in mission and campaign editing itself and this is our first goal. After we’ll see the first great user made campaign, we will probably give people some way to add [their] own models or even worlds to the game."
Recently, in a January 3 2002 interview with OFP.DE, Viktor Bocan, BIS' chief designer, made the following quasi-optimistic statements:
"There is nothing like an SDK for OFP, so it could not be released. There are some programs that allow people to make models, weapons, islands, etc. They will be released slowly in near future."
"What I said doesn't mean that an SDK or anything like that will be released in next few weeks. Not all tools will be released. Not all tools which we would like to release are ready for releasing. Not all tools will be released together. We will release them one after one over quite a long period of time."
"You have to understand that almost no tools were intended to be released when we created them - they were for internal use only. We have no documentation. Working with some parts of them is very difficult. You have to understand that we want to help people with making mods. But there is a lot of work that must be done first on these tools if we want to release them."
One such tool is called Oxygen. Another one is called Visitor. You can read a little bit about them in this FAQ item.