Android:
https://play.google.com/store/apps/details?id=de.blinkt.openvpn
IOS:
https://apps.apple.com/ru/app/openvpn-connect/id590379981
Mac OS:
http://openvpn.net/download/openvpn-connect-v3-macos.dmg
Windows:
64 bit: http://swupdate.openvpn.org/community/releases/OpenVPN-2.5.6-I601-amd64.msi
32 bit: http://swupdate.openvpn.org/community/releases/OpenVPN-2.5.6-I601-x86.msi
Recommented Win client: https://client.pritunl.com/