If you have access to iDRAC you can update the OS Collector and OS Drivers Pack using iDRAC GUI. You can traverse to Update page (Overview -> iDRAC Settings -> Update and Rollback) and upload these images. What iDRAC FW you currently have on the server?
I downloaded the latest version of the OS from Dell's "Drivers & Downloads" site (R620_Drivers-for-OS-Deployment_Application_W7W06_WN64_15.07.07_A00) and placed it on a USB. I changed the settings of the file to OS.exe on the USB to make it easy, and when I launched the Firmware update, selected "Local Drives", found my local drive, file path or update package path, I used the file on the USB (OS.exe) and hit next. I also, tried it with the full file name with and w/o the extension(.exe) ERROR OUTPUT: (SUP0513) "Enter Correct Path" IDK???  
Dell R620 OS Collector and OS Drivers Pack Version is set to "0"