Let us not approach this question from a worldbuilding point of view but the philosophical one.
we have two questions here: How would a god prove omnipotence and, the much bigger one: WHY?
How does a God NOT prove his omnipotence?
How would you have him prove his omnipotence? Pouring rain on the people? Strike them with lightnings? Burn them in fires or haunt them with illnesses? The opposite question is easier to answer: What cannot be counted as a prove of omnipotence? Short answer: About anything of the aforementioned.
We live in a time where basically anybody can purchase devices to strike others with "lightnings" and though they aren't used that way you could create them so that they would easily kill somebody and even make the remains (almost completely) "disappear".
Burning people with fire is nothing new anyway, so this doesn't prove much "potence" less even "omnipotence".
Illnesses? Well, we're not "perfect" at it yet but we can create our own illnesses already. Science will go further and get better at it (in healing and in creating them).
Rain? We cannot really make it rain at our bidding yet (close though). But some day science will manage this too. And drowning everybody will not really work either (no more deluge). Too many people have access to boats and even if many of them would die anyway, there would be surely some more beside the believers who'd survive.
So what is left??
Doesn't look good, does it? How can he still prove his omnipotence if about anything he would perform can (more or less) be done by humanity almost as well?
There's one thing humanity cannot overcome so far and probably never can completely: Death.
We are now left with two options, let's further explore them.
To be entirely immortal is already quite impressive (wouldn't mind that for myself either). However, does it prove omnipotence? No, it doesn't, because there's a caveat: Death remains a power beside the omnipotent God of which he could still be afraid. After all: What would happen if an immortal being dies? It sounds like a paradox but it is not. If immortality is a power that is needed to keep death at bay, then death itself (thought of as an entity) would still be stronger. If the immortal wouldn't have the power of immortality, he would die and remain dead for eternity. Death would prevail.
So not to die is still not a proof of omnipotence and chances are even that some day even humanity can attain this state.
What is the second option?
To die. It's that simple: He has to undergo the state of total unmight. He has to be stripped of every last spark of power he wields. If he is truly omnipotent, he can overcome it, otherwise he remains dead. In the latter case death remains the strongest of all powers (so to say: death thought of as an entity would be the omnipotent god), in the first he has proven that he is more powerful than the most powerful power (yeah...), he's proven to be stronger than death. It might not convince the last stubborn sceptic but at least it proves he's stronger than the strongest we can perceive (if, at all, we can perceive death).
So to prove your omnipotence or at least to prove to be the strongest and most powerful entity you have to undergo and overcome death. Simply avoiding it is not enough, you have to "beat" it.
But why would he want to prove it anyway???
That's now the actual question here. If he is omnipotent, he doesn't have to care a thing about what people do. Just let them do, he has the final word anyway.
The most enticing part about this question is the fact that we will never know why an omnipotent God would create beings able to turn their backs on him. The simple and easy way out-answer is delivered by Calvin, Luther, later Barth and others: Predestination (Karl Barth: "theatrum gloriae dei"). In my eyes a very unsatisfying answer - much less even an answer rather than a "workaround" to avoid the question.
We might have to further investigate the philosophical connection between love and freedom (Wolfhart Pannenberg did as far as I know, though I don't know where). But the question raised here is then: Why does an omnipotent being need other beings to love them and be loved back? Of course we can say that it was out of the free will of the omnipotent being but why would that being have that will?
We can answer the question of HOW with a certain accuracy, but we are entirely unable to ever find a satisfying answer to the WHY.