A bra and panties. Oliver felt like a total perv for taking these. But to be fair, there was some logic in it- he could wear other stuff on top of them, plus it didn't make much sense for a girl to be going around without any underwear. Slowly he undressed so he could put them on, which of course meant he had to strip naked. His T-shirt came off first, then his shoes, socks and jeans, and he stepped out of his boxers, staring at the reflection of a male body for probably the last time for a while.
He pulled the panties up his legs and did his best to get them over his crotch, and felt the ring make his finger tingle. He didn't even have to get the clothes to fit, he realized, the ring's magic was taking effect already. Frozen with amazement, he felt and saw his body's sex inverse itself.
His figure reshaped, narrow shoulders and curves and wide hips emerging as he got more petite. His body hair receded almost everywhere and his skin smoothed, so he appeared more supple and soft. His facial features became prettier, with soft round cheeks, a less firm jawline and puffier lips. His nipples swelled and grew sensitive, soft flesh slowly sprouting on his chest around them. His testicles shivered, and with two wet pops they moved up into his body and rearranged into ovaries, castrating him. His penis seemed to retract, as all but the head shrank away and the head turned into a sensitive nub, while under it his scrotum sank inwards and then split down its centre, forming vaginal lips.
What really stood out to Oliver as he changed was how he mentally seemed to turn from male to female too. A lot of instincts he'd had soon seemed strange as he became she. The biggest change was to his- or rather her- sexual desires. Instead of wanting to have his penis inside or rubbing at things, she wanted someone or something inside her vagina and for someone to rub or suck on her breasts.
Slowly she came down from an aroused high as her changes finished, and hastily dressed in her bra, panties and too-big clothes.