11:37 AM -- HBO tells TMZ, "We have confidence in the staff of Last Week Tonight and do not believe anything in the show this week violated Mr. Murray's or Murray Energy's rights."

John Oliver knew what was coming, and now he's got it -- a big fat lawsuit from the head of Murray Energy who apparently doesn't enjoy being called a "geriatric Dr. Evil" ... at least based on the docs.

For nearly a half hour on his show, "Last Week Tonight," Oliver went HAM on Robert E. Murray and his coal mining corporation, which is the nation's largest. The comedian tore into the coal industry in general, and mocked Murray specifically.

In the suit, Murray says the show was a "meticulously planned attempt to assassinate" Murray's character. He also says Oliver deliberately left out facts about an accident that killed 9 miners. Murray's insisted an earthquake caused it, not mining ... and claims Oliver ignored reports that backed his quake theory.

One part of the show focused on minuscule bonus checks some miners sent back to Murray, including one with the words "Eat s**t, Bob" written on it. According to the suit, the company's website was hacked, after Oliver's show aired, and flooded with "Eat s**t, Bob" messages.

Oliver acknowledged Murray's company warned it might sue. Oliver defiantly said, "I know you're probably going to sue me over this. But you know what? I stand by everything I said."