Obtaining OpenTTD
From #openttdcoop wiki
Obtaining OpenTTD
OpenTTD comes in a variety of versions. There is a stable version, which can be downloaded directly from the OpenTTD.org homepage. And there are the so called nightlies, which consist of the latest build of the night before (hence the name). These nightlies are development snapshots with the latest features added into them. Servers of #openttdcoop always run on a nightly version of OpenTTD, though not always the latest. To get the correct version to join our servers use the IRC command !download <your operating system of choice> in chat and it will provide a download link for the corresponding version that is currently in use on that server.
Valid options for <your operating system of choice> is: lin, lin64, osx, win32, win64, or win9x