Oleandra's mind was closed shut to Dumbledore's Legilimency; all he ever heard from her and her friends was static. Ginny, however, was an open book to him! Or so he thought…
In a contest of Legilimency and Occlumency, the one with the highest mastery over mind magic would win; that was the generally held consensus.
A novice Legilimens needed to say the incantation out loud, use a wand and maintain direct eye contact. However, anyone would put up their guard if they heard someone announce their intentions to read their mind! A true master only required eye contact and could move around unnoticed in their target's mind, and Dumbledore was one such man.
A novice Occlumens could at best block a weak attempt at mental intrusion if they were on their guard. As one mastered Occlumency, one would eventually be able to guard against stronger attacks, as well as detect signs of mental intrusion even when they weren't expecting an attack. A true master, however, could create a false world of memories, and only allow the Legilimens to read what he wanted. In theory, a grandmaster could even trap another's mind forever within their own…
Tom Riddle had mastered Occlumency by his fifth year; otherwise, he'd have been arrested a long time ago for the murder of a student when he had opened the Chamber of Secrets and freed the Basilisk. As Oleandra constructed her story, Ginny built false memories for Dumbledore to read!
Incidentally, Oleandra and the rest of her friends had no idea Legilimency and Occlumency even existed; they had never needed to learn any of it, since their minds had been closed to mental attacks from the start! (There was also the fact that Dumbledore didn't exactly go around advertising that he intruded upon his students' privacy all the time for the fun of it!)
"Miss Weasley," said Dumbledore after a while. "How is it that you were in Hogsmeade to begin with? Second years aren't allowed to the village, surely you should have known better?"
"Ever since I was a little girl," Ginny said in a low voice, "I've watched my big brothers go off to Hogwarts, one by one. I knew one day it'd be my turn, but it couldn't come fast enough. They always told wonderful stories about Hogsmeade, so I asked Fred and George for a way to sneak out… I'm sorry…"
Ginny deliberately avoided mentioning Harry Potter in her story. But in her mind, she was constructing a fake thought for Dumbledore to find on purpose: He can't know about Harry taking me. It's not his fault, I forced him to take me. Her feelings of love and of wanting to protect Harry were genuine, and that was something that Dumbledore knew couldn't be faked. Truths within lies within truths.
Dumbledore never expected Ginny to be a master Occlumens, and so, she held the initiative to lead him towards the conclusion she wanted him to find. Dumbledore also was actually very biased; he was using trustworthy Ginny Weasley as a baseline to compare her memories against the Slytherin Oleandra's story to find faults in it.
"I lost count of the time," Oleandra continued her story. "He had us in a strange room for days, at least; Ginny called it the Room of Requirement. Black wasn't there all the time. Eventually, I managed to cast a spell wandlessly and freed Draco, Daphne, Tracey and Stacey. But at that moment, Black came back, but Draco led the others through a cabinet (it had been Draco's idea to add this to the story) and they somehow disappeared. I don't know where they went."
"You'll be happy to know that your friends and sister are all safe and sound. I suspect they found a broken Vanishing Cabinet, which whisked them to another part of the castle," said Professor Dumbledore pleasantly, looking straight into Ginny's eyes. "Please, do go on."
Ginny had to keep herself from rolling her eyes. Could the old man be any more obvious? She continued producing fake memories, taking care to obscure 'Sirius Black's' face for plausible deniability.
"He was mad; really mad," Oleandra sniffled. "He asked the same questions again and again, and he hurt me, and I kept telling him I didn't know…"
At this point, she had unknowingly started shedding warm tears at the memory of her torture at the hands of the Dusk-Elf agent. This wasn't a ploy to fool Dumbledore; she was genuinely distraught. Fear, anger and adrenaline had carried her this far, but now she had gone way past her limits.
"What did he want to know?" Dumbledore urged her to answer. Just as he was prejudiced against Slytherin, he was similarly biased against Sirius, who was part of the Black family, notorious blood supremacists. His heart told him to trust that Sirius was innocent, but his mind told him not to believe in Sirius!
"That's enough, Albus," Madam Pomfrey said sharply. "Can't you see she's shaking? The girl needs her rest. Out! Out, I said!"
And thus, Professor Dumbledore was unceremoniously kicked out of the Hospital Wing.
"Inconsiderate old fool," Madam Pomfrey muttered under her breath. "I swear, someone's going to die, one day, and no amount of healing can bring someone back from that…"
"S'cuse me, what day is it today?" Oleandra asked, wiping her tears with a tissue that Ginny offered her.
"Why, it's Christmas Eve!" said Madam Pomfrey. "Poor girl…"
According to the contents of Daphne's telepathic message: after waiting for a while in the Room of Requirements, she understood that something had gone wrong on Oleandra's side. At the time, she'd led the others out of the room, only to be found by patrolling prefects after reaching the dungeons; that had been yesterday.
Oleandra counted the days: Daphne and the others had arrived yesterday, despite having spent eight days in Nidavellir, which meant… time passed about twice as slowly in Nidavellir. She had no idea how long the Dusk Elves had kept her unconscious, but going with the math, she had to have stayed two additional days.
"I guess I'm the older sister now," Oleandra chuckled to herself. "By about one day…"
"What was that, dear?" Madam Pomfrey said distractedly. "Now, what I'd say you need most right now are your friends and family, so I'll be healing you throughout the night so that you can be together with them on Christmas Day."
"Thank you so much," said Oleandra gratefully.
"It's a little early to be thanking me," said Madam Pomfrey wryly. "By the time I'm done healing you, you'll be cursing my name."
Oleandra gulped.