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There are several choices to be made when configuring OpenVPN.

Routed or Bridged?

A routed VPN configuration means that clients will receive IP addresses on a different subnet than the one you're using for your network, and the VPN server will route requests between the subnets.

A bridged VPN configuration creates an Ethernet bridge between the clients and server. All clients will have IP addresses on the same subnet as the server and other hosts on the server's network.

Numbering private subnets

The private subnets used for your local network as well as (if using the routed method above) the VPN client IP pool need to be sufficiently unique to lower the probability that a remote site's private network is using an overlapping or coinciding address space. A popular subnet in use because of default router configurations is 192.168.1.0/24. Attempting to establish a connection to a site using that subnet from another site using that subnet will result in routing conflicts. A request to 192.168.1.100 is ambiguous under this configuration. For that reason, OpenVPN recommends you configure your local network (and VPN client subnet) to be address spaces somewhere within the large 10.0.0.0/8 range.