It's simply not possible to enforce using a certain application to open your document. By their nature, files may be opened by any and every application. Whether or not the application understands the data is another matter.
Based on the fact that opening your sensitive document in OpenOffice reveals the sensitive information, your file is not secure, and Excel is merely providing the illusion of security. It's like locking your car doors but leaving the windows rolled down. You still use your key to "lock" your car, but there's almost no point if a theif can just reach through your window to access your valuables.
I propose instead of using Excel-specific "password-protection," that you properly encrypt the sensitive data using something like AxEncrypt. This will ensure that only those with the password will be able to access the data.
AxEncrypt typically works on whole files, and not partial files, so I would propose having a sensitive version of the file containing the "secret" tab, which is encrypted, and a non-sensitive version of the file with the "secret" tab omitted.