What internal drives do you have installed in this iMac?
What is the highest level of macOS ever installed on this iMac?
When you install a new drive in a computer, you need to use Disk Utility to erase the new drive before it can be used for anything. If you are trying to install macOS 10.14+, then you will need to erase the whole physical SSD as GUID partition and APFS (top option). Within Disk Utility you may need to click "View" and select "Show All Devices" so that the physical drives appear on the left pane of Disk Utility.
If you are not seeing the physical internal OWC SSD, then either you have a bad connection or the SSD is bad. Make sure you connected both the power and data cables to the SSD. If the OWC SSD upgrade kit included a temperature sensor adapter, then make sure you have it connected to the SSD and the source cables as well.
The OWC Mercury 6G SSD is a SATA based drive, so you can test this drive externally using a USB to SATA Adapter, drive dock, or enclosure. You can connect it externally to any computer even a Windows computer to verify the physical SSD can be seen.
