It appears that there are two different installation folders you are trying to use.are you doing any side by side installation and trying to use two different version at the same time?
check these links
Sorry if my post is confusing. I am not trying to use 2 folders.
I am just trying to access my Oracle Connection via Tableau Prep. However, it seems that Tableau Prep can’t find the oci.dll file.
Sorry, I realised my own mistake. I wasn't downloading the right driver when Tableau pointed me to the page to download it.
There is a Separate Driver to Download to Tableau Prep. Need to scroll down on the website, pass all the various versions of Tableau Desktop Drivers before you can see the "Tableau Prep: 2018.1.1". To be exact, it is currently below "Tableau Desktop: 8.2" driver