Below is the clue that suggests that Oberon Sexton is the Domino Killer:
Oberon’s last name Sexton is defined as a caretaker of graveyard, and often also as bell-ringer and gravedigger.
All the Black Glove victims wear domino masks. In the game Mexican Train, unused dominoes are kept in the “bone yard”; Bone yard is synonymous to graveyard, which relates to Oberon and his alias.
In Batman and Robin #13 Joker reveals that since the death of "his Batman" he is no longer the "Clown Prince" he once was and took on the identity of Oberon Sexton to gain the new Batman's trust.
Most likely, the detective persona was a conscious façade. It's also possible that Joker’s Multiple Personality Disorder led to a detective personality.
However, Dick Grayson the Batman found out that Oberon Sexton is the Joker as he solved the crimes pertaining to the murders of the four deceased Black Glove members having clues all designed to hint at jokes.
1. The Oil Sheikh, the peanuts and the restroom.
Connection:
Rich Oil Sheikh Joke
A rich oil sheikh was visiting the White House to discuss the oil trade with political bigwigs. His personal servant was attending to the sheikh's every need, as was customary and expected. The sheikh, never having tasted salted peanuts before, took an instant liking to them, and began munching them like you wouldn't believe. Naturally, the salt made the sheikh thirsty, and he called to his servant to get him some water. His servant left the room and came back with a glass of nice, cold water. Munching some more on the peanuts, the sheikh got thirsty again, and called to his servant to get him another glass of water. The servant immediately complied with his master's wishes, and returned in a jiffy with another glass of water. Really taken by the peanuts, the sheikh virtually stuffed himself on them, and got a healthy thirst to match. He called to his servant to get him some more water, and off the servant went. This time, he came back empty-handed. The servant apologized profusely, and begged his master's forgiveness. Furious, the sheikh yelled and screamed at his servant, 'You son of a flea-infested camel, why can't you bring me my water?!?' 'But Master', begged the servant, 'I cannot bring you your water because a white man sits on the well...'
http://www.opendb.net/element/15152.php
2. Cardinal Magi strangled by his own dog collar.
Connection:
There are jokes about cardinals’ "white collar crimes” pertaining to the misdeeds of the cardinals.
But this one is the closest.
Cardinals Jokes
Two boys were playing football in a Dallas area park when one is attacked by a Rottweiler. Thinking quickly, the other boy rips off a plank of the nearby fence, wedges it down the dog's collar and twists, breaking the dog's neck. A reporter who was strolling by sees the incident, and rushes over to interview the boy.
"Cowboys Fan Saves Friend From Vicious Animal," she starts writing in her notebook.
"But I'm not a Cowboys fan," the boy replies.
"Redskins Fan Rescues Friend From Horrific Attack," the reporter starts again.
"I'm not a Redskins fan either," the boy says. "I'm a Cardinals fan."
The reporter writes on a new sheet in her notebook: "Redneck Bastard Kills Family Pet."
http://anti-cardinals.blogspot.com/2009/10/crapnals-jokes-from-old-site.html
3. The Newspaper Tycoon with the Heart Attack and a Young Mistress.
I haven’t found any joke/jokes referring to this one. But maybe, just maybe, an old man having a heart attack while having sex with his young mistress is funny enough for the joker.
4. Remains of the Russian general Malenkov found inside an Alligator.
We’ll remember that the Russian general Malenkov was killed by Joker in Batman #681. In Batman and Robin #10 Oberon (who is the Joker in disguise) told Batman that Malenkov’s remains is found inside an alligator.
Connection:
One of Russia’s combat helicopters was named ‘Alligator’, but it is more likely that the alligator being described by Oberon is the reptile one.
http://www.enemyforces.net/helicopters/ka52.htm
Also, there’s this "bar joke" about a man and an alligator which is more likely have the closer connection:
Alligator in the Bar
A guy walks into a bar with a pet alligator by his side. He puts the alligator up on the bar. He turns to the astonished patrons. "I'll make you a deal. I'll open this alligator's mouth and place my genitals inside. The gator will close his mouth for one minute. He'll then open his mouth and I'll remove my unit unscathed. In return for witnessing this spectacle, each of you will buy me a drink." The crowd murmured their approval. The man stood up on the bar, dropped his trousers, placed his privates in the alligator's open mouth. The gator closed his mouth as the crowd gasped. After a minute, the man grabbed a beer bottle and rapped the alligator hard on the top of its head. The gator opened his mouth and the man removed his genitals - unscathed as promised.
The crowd cheered and the first of his free drinks was delivered. The man stood up again and made another offer. "I'll pay anyone $100 who's willing to give it a try."
A hush fell over the crowd. After a while, a hand went up in the back of the bar. A woman timidly spoke up. "I'll try, but you have to promise not to hit me on the head with the beer bottle."
href=http://www.eszlinger.com/jokes/barjokes.html
Have you spot the connection? No?
Penis is also referred to as genital. In the joke the man puts his penis, his genital into the alligator’s mouth. What’s the connection to Joker killing Gen. Malenkov?
The Joker put the general (ge~ne~tal) into an alligator’s mouth.
That’s it.
There are plenty of clues that suggest who Oberon Sexton and who the Domino killer is and they’re all hiding in plain sight.
Dick found it, obviously we did not.
