I just erased my hard drive on Imac ( mid 2011). How do I install OS?
Need help on reinstalling os after erasing the HD
iMac 21.5″, macOS 10.13
If this iMac had macOS 10.12.6 or 10.13 installed at some point, then try booting into Internet Recovery Mode to access the online macOS installer:
https://support.apple.com/kb/HT204904
Otherwise, if you have access to another Mac which is compatible with macOS 10.11 or 10.13, then you can create a bootable macOS USB installer:
https://support.apple.com/en-us/HT201372