.Exe file keeps being opened as text file

An .exe file is a Windows executed file and won't open unless you're running Windows.

.exe files are usually executable files on Windows: they won't run on a Mac. Very occasionally a file which doesn't have an extension will get labelled as .exe because the Mac doesn't know how to handle it - this can happen with old AppleWorks files if they don't have the extension: adding the correct extension corrects the problem.

However from what you say this may be an actual application imported from a Windows machines? If so it won't run under OSS. You would have to use one of the methods for running Windows - Boot Camp, Parallels, or VmWare Fusion (which require you to have a copy of a Windows OS).

Nope, it hasn't worked before, because .exe files don't run on Macs any more than Mac apps run on Windows. I'm guessing you have installed OpenOffice before and just downloaded the installer again, but got the wrong one. If you didn't download it on a Mac, it would have auto-detected that you were using Windows and downloaded the Windows version. Either download it on a Mac, or use this page to download:

http://download.openoffice.org/other.html