Is it difficult to switch from United to Lufthansa airlines at Chicago airport?

International flight from OKC to Germany thru Chicago. Layover.

Only if you can't read. If you can read, the signage makes it easy to find where you want to go.

Helps if you allow enough time so that you do not have to run to next flight.
The O'Hare Airport Transit System (ATS or "people mover) is closed 24/7 to finalize modernization efforts and expansion of the system. There's a bus system to move you around inside the security zone. Follow the signs.

Lufthansa departs from Terminal 5

You will probably arrive at Terminal 2
Depending on method you use to get between terminals you can avoid a security screening wait.

As many things can happen to delay your arrival in Chicago not a good idea to plan on pushing the limits to be last on board your Overseas flight.

there's a wait for the bus to take you to terminal five.

Yes. O'Hare is the worst airport to change from domestic to international and vice versa. You have to change terminals- which means waiting for a bus and then going through the whole security thing again I hope you have plenty of time between flights.

It may be time-consuming, but as long as you can read and have allowed yourself a realistic amount of time to do this in, it is not 'difficult'.