Which in turn was quite appropiate, as Shatner had arrived on horseback – George probably knew that was to be his parting shot the instant he saw that.
Right. I did mention Shatner rode onstage on his horse.
I actually found myself doing the fanning-myself-off motion while trying (and failing) to stifle my laughter. That was incredibly awesome.
“Oh My…”
George Takei is now the official announcer for the Howard Stern Show on satellite radio, sits in on the show for a week several times a year, and this is one of his many hilarious expressions the show has popularized. He is a funny, funny man.
All I can say is “Hee!”
I think George Takei first said, “Oh my!” when he saw John Wayne in his green beret uniform. He probably mistook him for a bear. 
In regards to the OP, I’m pretty sure I saw the episode of Howard’s old E! show on which he first said it. Must have been early to mid nineties. Takei was dealing with a female caller who believed that his bad mouthing of Shatner was just due to jealousy and she said, “…I believe you’re just a sad, evil person”, to which Takei famously replied, “Oh my…” in his very dry, unemotional way. It got a huge laugh from everyone in the booth (I think Takei was on a satellite feed).
I used to watch Howard’s E! show a lot (sadly I couldn’t receive his radio show) and I remember it was only after this episode that the sound bite started appearing. So I think this is where it spontaneously originated.
George seems to have had an effect.
This July, Tim Hardaway (that “basketball player guy or something”) was the first person to sign on to a petition seeking to legalize gay marriage in Florida. He also twitted nice things when NBA player Jason Collins came out.
“Hold on - homophobic means anti-gay? Then I meant the other thing.”