Momo's magic of course!

"I suppose you could spawn a swimsuit for Ochaco, couldn't you?" Izuku asked Momo.

"Of course, Master, it only takes one wish!" Momo replied. "Swimsuits are pretty sexualized, so they're easy enough. Or I could just use my Quirk."

"Nah, save your lipids for things your magic can't produce," Izuku denied logically. After all, Momo's magic pool was basically limitless.

"Okay then, in that case, I'd like a new swimsuit myself," Momo nodded. "What color would you like yours to be, Ochaco?" she asked. "And would you like a one piece, or a bikini?"

"One piece please," Ochaco grinned. "And pink sounds good."

Momo just nodded, a mischievous glint in her eyes. "Okay then!"

Izuku, despite seeing the look in Momo's eyes, was a bit too curious to see where she was going to act on it. "Alright, Momo, I wish for you to create a swimsuit for yourself as you want it, and one for Ochaco within the specifications she listed," he said. "Right sizes too, obviously."

"As you wish, Master," Momo grinned, and snapped her fingers. Instantly, two swimsuits appeared, one red, the other pink... And they were both sling bikinis that would cover the important bits, but basically nothing else.

"...I should have seen that coming," Izuku remarked with a blush, Ochaco also staring them, blushing heavily as well. "...it IS technically a one piece, I guess we gave her a little too much wiggle room."

Ochaco picked up the pink sling bikini, blushing heavily. "Are you serious?" she asked the giggling Momo.

"You only said "one piece," you said nothing about the amount of skin coverage!" she laughed.

"It was implied!" Ochaco squeaked, before looking at Izuku. "...do you think I should wear this?" she asked after a moment of hesitation, thinking things over.

Izuku blinked at that, before it clicked. Despite Ochaco being initially taken aback, she was seeing this as an opportunity to do something sexy for Izuku, and was basically asking her to make her ear it, without asking her to make her wear it. "...go for it. I think you'd look great in it," he took the plunge, despite a heavy blush on his face.

"You're the boss!" Ochaco grinned, and despite her embarrassment at the thought of being seen in public in the swimsuit, started undressing in front of Izuku and Momo to put it on, swapping out her underwear for the sling bikini. Momo of course dispelled her own clothes before slipping into her own, looking all too pleased with herself.

Once clad in them, Momo and Ochaco grinned, gazing at Izuku eagerly as they stood beside each other. "So... What do you think, Master?" Ochaco asked, her blush adding to her cuteness.