Loli in commercially-released 18+ OVA is fairly uncommon compared to its density in 18+ manga; a lot of stuff is either low-quality or 'does this even count as loli?' Many multi-girl series also have loli/near-loli scenes, but it'd be impractical to try and list all of those. I'm going off of memory or very quick+arbitrary judgment calls while making this list so forgive me if there's something not loli enough where it shouldn't be. This is definitely not meant to be a comprehensive or definitive list, but should cover most of the important ones.
OVAs with a lot of loli:
300 En no Otsukiai
A-Size Classmate
Chicchana Onaka
Choisuji
Doki Doki Little Ooya-san
Ecchi na Shintai Sokutei
Garden
Hajimete no Orusuban
Harumi-chan no Oita
Ichinen Buri no
Implicity
Little Monica Monogatari
Lo Re Pako Sukusuku Mizuki-chan
Nama Lo Re Furachimono
Nama Lo Re Nama Kemono
Namanaka Hyaku Percent
O-Parts Oman
Onii-chan Asa made Zutto Gyutte Shite
Oyasumi Sex
Peropero Teacher
Seikatsu Shidou
Shoujo Kyouiku + Shoujo Kyouiku RE
Shoujo Ramune
Shoukoujo the Animation
Shoyonoido Mako-chan/Shownoid Mako-chan
Suki de Suki de Suki de
Tiny Evil
Toromitsu Musume no Hitou Service
Toshi Densetsu Series
Yunyun Paradise
Loli-ish, or has a loli-ish girl in multiple scenes; your mileage will vary:
Angel Core
Binetsu Hime Zange no Shou
Cream Lemon
Ecchies
Enjo Kouhai
Gaki ni Modotte Yarinaoshi
Hitoriga
Ichigo Chocola Flavor
Imako System
Imouto
Imouto de Ikou
Imouto Jiru[Expand Post] Imouto Paradise series
Kanojo x Kanojo x Kanojo
Lolita Anime Nikkatsu Video (Uchiyama Aki)
Lovely Heart
Mahou no Rouge Lipstick
Makai Tenshi Djibril series
Momoiro Bouenkyou
My Imouto Koakuma na A-Cup
Natsumushi
Sono Hanabira ni Kuchizuke wo
Some special mentions:
Rune's Pharmacy ep3 2nd half - only loli scene in the series, but if you're a totfag this is one of the only things around that'd likely be in your strike zone
Muchi Muchi Kyousei Seichouchuu - not at all loli technically, but all the girls are literal kindergarteners who retain that mental age after getting physically aged up
Dream Hunter Rem - started as hentai but then became a mainstream OVA after 1 episode. Focuses on a cute little girl in a bikini who fights dream monsters. Shilling it mostly because it's fun.
I can also talk about non-18+ anime that have a loli focus, if anyone cares.