In Star Wars Episode IV: A New Hope, Luke Skywalker knew Obi-Wan Kenobi under the name of "Ben" :
Well, I stumbled across a recording while I was cleaning him. He says that he belongs to someone named Obi-Wan Kenobi. I thought he might have meant old Ben. Do you know what he's talking about?
Where did this "Ben" nickname/alias/false identity come from?
This was explained in a John Jackson Miller's 2013 EU/Legends novel "Kenobi", covering Obi-Wan's life on Tatooine:
He picked a name Ben because, as he explained in his meditation attempt to contact Qui-Gon:
Meeting her {{a teenage girl character named Kallie - DVK }} gave me a chance to finally use the name I chose. You’ll like this: Ben. I had seen it on the map at the property office in Bestine—there’s some mesa by that name. Satine used to call me that—it was a private thing. I like the sound of it.
His last name (Kenobi), he was NOT intending to use in the first place, and introduced himself as "Ben".
BUT... samesaid Kallie (a teen with a crush on him) sneaked around his hut and heard him use it when he was attempting to meditate and talk to Qui-Gon Jin through the force... aloud.
But I guess I’d rather see, and do nothing, than not see at all. I’m missing so much of what’s going on in other places. I can’t live blindfolded. It’s not really the Kenobi way.
...
I guess I can become “the Intrepid Kenobi” if I must.
...
… wait.
Hold on.
…
Someone’s here!
Being an airheaded teen with a crush, obviously the very next morning, everyone in the settler community around knew his name (Ben Kenobi).
“KENOBI.”
Annileen rubbed her eyes. “What?”
“Kenobi,” Kallie said, beaming across a cup of blue milk. “That’s his name.”
“What?” Annileen glowered at her daughter. Annileen had risen at the usual deathly hour, remembering Kallie’s absence from the night before. But the girl was here, now, at the breakfast table in the family quarters. Wide awake—and positively quivering with excitement.
“It’s his name,” Kallie said.
“Name? Whose name?”
“Ben’s name!”
Annileen stepped forward. “How do you know that?”
Jabe called from the larder. “She went to his house!”
As such, that was how people on Tatooine came to know him by.
He was OK with it (well, resigned but not panicked), because apparently there were gazillion Kenobis on Tatooine already, it luckily was a common last name.
{{ continuing from the earlier quote when Kallie tells her mother }} Kallie took her mother’s vacant expression as a signal to go on. “His name’s Kenobi.”
“Someone called him that?”
“He called himself that,” Kallie said. “I couldn’t see who he was talking to—but he said it. He was just sitting there, talking about his day, and the people he’d met, and the Tuskens.”
Annileen looked at her with skepticism. “You’re not just making this up?” She tried the name out. “ ‘Ben Kenobi.’ ” She’d known other customers by the surname over the years, and seen it spelled several different ways on her receipts.
Jabe dabbed up the last of the gravy on his plate. “Lot of Kenobis around. There’s the couple near Bildor’s Canyon.”
“There was that podracer pilot,” Kallie interjected, excitedly.
... and later in the chapter, when Ben shows up at the store which is owned by that family:
Orrin squinted. Well, I’ll be, he thought. He stood tall and waved. “Hey, Kenobi! Ben Kenobi!”
For a moment, the hooded man appeared to disappear back behind the dune. But when Orrin charged up the rise, he found Ben kneeling to adjust his boot. “I thought that was you,” Orrin said.
Ben stood. As Orrin shook his hand vigorously, Ben said, “I’m sorry. I thought I heard you say—”
“Ben Kenobi—that’s your name, isn’t it?”
Ben looked down, around, and back to Orrin. “Yes, but—”
“But what?” Orrin smiled.
“I was just curious how you heard it.”
“Oh!” Orrin laughed loudly and slapped Ben on the back. “You’ll find out when you go inside.” He turned toward the store, coaxing Ben along.
...
A high-pitched shout came from the Claim. “Ben!”
...
“I think you’re going in, brother.” Orrin put his hand on Ben’s shoulder. “Kenobi. There was a Kenobi down around Arnthout, sold damper coils for repulsorlifts. You any relation?”
“Anything’s possible.” Ben smiled narrowly, teeth clenched together, as Orrin pushed him toward the doorway.
...
... Ben said. Then he stepped inside the Claim.
“Kenobi!”
The man’s eyes widened at the sound of the name called out by the clutch of people at the end of the bar. But Orrin guided him inside.
Leelee Pace looked back from her packages and waved. “Hey there, Ben Kenobi!”
By the sundries aisle, Doc Mell told his child, “There’s that Ben Kenobi. I think he’s a doctor, too!”
...
Ben looked at Annileen—and then back at Orrin. The visitor was slightly bemused, Orrin was glad to see. It happened this way all the time. People came to the oasis wanting to keep to themselves, for whatever reason—not knowing that small-town life made privacy completely impossible. ... “You’re the fellow from yesterday,” the old man said, tugging at the sleeve of Ben’s robe like a tailor droid checking a seam. “You’re a Kenobi?”
Ben pulled his sleeve back. “I—”
“Hired a Kenobi once. Gormel, they called him. Thief. Stank of spice all the time. I fired him quicker than you can say your name.”... Ben said, his eyes settling on Kallie as she chattered with some teenage friends. “I think I know what happened now.”
“I’m sorry,” Annileen said. “Kallie came looking for us and ended up snooping. I’m mortified.”
Orrin gestured for others to leave the bar, making room for him and Ben. “Gossip gets around pretty fast where there’s not a lot goin’ on.”
.... and later when meditating (this time, silently!) to Qui-Gon, Kenobi notes:
It doesn’t seem that the girl overheard anything too revealing—or saw me holding Anakin’s lightsaber. The Force was with me there. And there are apparently a number of people around whose names sound like Kenobi, so nobody’s drawn the connection. Yet.
Obi-wan is a fairly distinctive name, and Vader knew he had survived Order 66.
He could hardly call himself Obi-wan Kenobi any longer. That said, he needed a link to his previous name, in the event that he was needed (as he was, by Bail Organa, years later).
Last names, in the SW galaxy as well as ours, tend to be firmer - a person's last name rarely changes outside of big events (such as marriage). First names are much more malleable.
Common names are also more frequently used alone. He wasn't even usually called 'Ben Kenobi' - he was called 'Old Ben' most commonly.
'Ben' is a close relative, phonetically, of his given first name, but common enough to not raise eyebrows. If his name DID filter into official communications somehow (such as in an official incident report) he would likely have been referred to by the citizenry as "Old Ben" or 'the hermit'.
His seclusion and the relative unimportance of Tatooine helped him hide behind a relatively flimsy false name, while still remaining locatable to his allies.
We have confirmation from the (canon) Star Wars: Absolutely Everything You Need to Know, Updated and Expanded edition that Obi-Wan used the nickname 'Ben' to evade the Empire's spies.
Q: Why is Obi-Wan called "Ben" on Tatooine?
A: Obi-Wan uses his old nickname so that he won't be found by the Empire, which is hunting down Jedi survivors after the Clone Wars.
I know this is a late response, but the name "Ben" is given an origin in the Season 4 story arc of the Clone Wars "Obi-Wan Undercover." When Obi-Wan goes undercover, his codename is Ben. This codename is known by Windu and Yoda but not Anakin, which makes it convenient enough to avoid detection from Imperial Forces because Anakin is not aware of any codenames Obi-Wan might use.
It is simply a contraction of Obi-Wan (in which one can easily see the name "Ben"). That is what I (without even much thought) concluded when I saw this movie at age 13. After all, what sense is there to any nickname that is attached to any given name.
He went by "Ben" because in the book 'Kenobi' he says he chose the name Ben because it was something Satine Kryze called him, and that he liked it because it was something personal.
I did a thorough research on why Obi-Wan turned to Ben, I listed all of the possible hypothesis, or at least I think I did, just reply if I forgot any, although I think that it is complete and I'm happy with it.
Since all info must be available in case the link goes dead, I summed the reasons up:
Reason 1: Obi Wan Kenobi used the Alias Ben for Infiltration (As seen on The Clone Wars)
Reason 2: Ben Could be Obi Wan Kenobi's Birth Name (OB-1 KNB Droid Theory)
Reason 3: He Chose to Tell a Girl his name was Ben (As he told Qui Gon's Force Ghost on his Tent)
It is one of these reasons, although opinions diverge on the "true" reason they are often just variations of these 3.
I don't believe that it was ever explained where Ben comes from.
Obi-Wan introduced himself to Owen and Beru Lars as “Ben Kenobi” at the time he handed them baby Luke and informed them that Luke was Anakin and Padme’s (orphaned) son. Ben Kenobi may have informed them that he was a Jedi ‘like Anakin’, or maybe they just intuited as much by his appearance and demeanor. When Luke mentioned both names Obi-Wan and Ben Kenobi, just being familiar with the name ‘Ben Kenobi’ could have been enough to set Owen off.
