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I setup an openvpn server with static key (certificate mode is not usable due to DPI at the national gateway), but I cannot successfully change the DNS automatically after the connection. I searched over the Internet and SE, and every one suggests the use dhcp-option.

I tried to add this line to client.ovpn

dhcp-option DNS 8.8.8.8

There is no effect. I tried to add this line in server conf

push "dhcp-option DNS 8.8.8.8"

Neither has any effect.

In fact, according to the manual,

--dhcp-option type [parm]

Set extended TAP-Win32 TCP/IP properties, must be used with --ip-win32
dynamic or --ip-win32 adaptive.

But my client is a Mac machine, server Linux. Any solutions to the problem?

On a Linux system, you need to run an external script.

Here is the documentation:
https://wiki.archlinux.org/index.php/OpenVPN#DNS

Scripts are found on here or newer Linux version here and you can call them by adding this on the openvpn client configuration:

script-security 2

up /etc/openvpn/update-resolv-conf

down /etc/openvpn/update-resolv-conf

I'm no expert, but from reading the man page below your quote:

--dhcp-option type [parm]

...

Note that if --dhcp-option is pushed via --push to a non-windows
client, the option will be saved in the client's environment before
the up script is called, under the
name "foreign_option_{n}".

and under foreign_option_{n}:

foreign_option_{n}

An option pushed via --push to a client which does
not natively support it, such as --dhcp-option on a non-Windows
system, will be recorded to this environmental variable sequence prior
to --up script execution.

So the command line openvpn client doesn't automatically modify the DNS setup on your OSX machine after establishing the connection. But since the DNS options are saved to the client's environment, you can specify a script to append the pushed servers to the system's current DNS config with the --up option:

--up cmd

Run command cmd after successful TUN/TAP device open (pre --user UID change).

cmd consists of a path to script (or executable program), optionally followed by arguments. The path and arguments may be
single- or double-quoted and/or escaped using
a backslash, and should be separated by one or more spaces.

E.g. Tunnelblick also does this by utilizing this script.

Another solution might be a script that was posted to the openvpn mailing list at DNS for OS X clients - the Definitive Guide.

I had the same problem with both Linux OS (server and client), and I solved itm installing dnsmasq on server that was missing

so my steps:

apt-get install dnsmasq

in server.conf

push "redirect-gateway def1"
push "dhcp-option DNS 8.8.8.8"
push "dhcp-option DNS 8.8.4.4"


And I could connect and browse domains through the server IP , connecting with NetWorkManager with DNS sets to automatic

Add this command to your client side conf file..

# put actual dns name here

dhcp-option DNS 10.11.12.13 

It will definitely work..