If you can obtain a copy of TweakUI from the Microsoft site you can use it to reverse your registry tweak, but I'm not sure if the version for Windows 98 is still available.
If you look at the files in the root directory of the CD you should find a file called AUTORUN.INF. If you use QuickView to look inside this file you will see something like the results below. These were obtained from the Times newspaper "The Month" CD.
The line starting with "open=" shows the file needed to run to start the installation of your oOffice CD.