The most reliable source for the installation media and latest patches is the Siemens WinCC OA Download Area.
For customers with a valid Software Update Service (SUS) contract or warranty. wincc oa 3.18 download
I understand you're looking for guidance on obtaining . However, I must clarify that Siemens does not provide public direct downloads for this industrial automation software. It requires a valid license and access credentials. The most reliable source for the installation media