Start petting her.

Izuku smiled, having Momo get up off his lap, and getting off the chair to get on his knees next to Ochaco as she approached him, still on all fours. He extended a hand towards her head, and began to pet her ears, giving them scratches. She gave a small jolt, before purring louder. "Oh! Nyah! That feels great..." she purred, pressing her head towards his hand the way an actual cat would when getting pleasant scratches. "More..."

He obliged, continuing to give her some scratches, and moved his other hand to start stroking along her back, making her jolt slightly again, before pressing her back against his hand. "Does that feel nice?" he asked her.

"Oh yeah, Master... Keep doing that, please..." Ochaco continued to purr, acting more and more catlike. "...wow, I'm either really getting into acting like a cat, or you added catlike instincts to the mix, Nyah..."

"A bit of both," Izuku chuckled, continuing to stroke her back, and scratch her head.

"...so, am I still your ****, or am I your pet?" Ochaco asked through her purrs.

"...a bit of both," Izuku repeated, getting a giggle out of Ochaco, and a snort from Momo. He then changed his strokes along her back to light scratching along it, intensifying her sensations.

"Ah, ahn, oh... Nyah..." Ochaco mewed as her front half sank to the floor, raising her rear a bit, allowing Izuku easy access to scratch near the base of her tail. "Oh yeah, keep it up..." she purred, her eyes closed in contentment as she nuzzled her cheek against his foot.