Rifts is a post-apocalyptic giant robots firing missiles at demons game, which isn’t exactly the kind of thing you think of for OSR or D&D, but Rifts: Atlantis is readily adaptable to the OSR. Atlantis has relatively little of the fascists and giant robots world-building that characterizes most of Rifts but rather is sword and planet with a hint of Lovecraft. That sort of thing seems anomalous from the high fantasy perspective that dominates D&D now, but it characterized a lot of the Appendix N literature and was a strong theme in old school D&D and OSR, from “Expedition to Barrier Peaks” to Hyperborea. The Rifts rules are pretty different from D&D so Atlantis is not exactly ready to run out of the box as OSR, but it has great potential as adaptation material for a major faction in your campaign world. I’ll first give a synopsis and then some ideas on adaptation.
The basic premise of Rifts: Atlantis is that an interdimensional alien confederation has established a territorial base for slave raids where they capture people and then sell them off in other dimensions, often after altering them with magical tattoos or magical parasites. The alien confederation are led by truly inhuman Lovecraft type entities, then about a dozen types of more or less humanoid aliens, and finally actual people at the bottom of the org chart. Much as real world empires are usually named for the numerically small nation at the top rather than the more numerous but diverse peoples they rule, the aliens are called the Slugorth even though only the handful of Lovecraft entities are actually Slugorth in the limited sense.
I wouldn’t adapt the setting wholesale in part because Atlantis in the game is huge; about the size of the continental United States. Rather I would make Atlantis the reasonably sized evil kingdom that threatens the rest of the region, like Iuz in Greyhawk. It can be on the mainland or an island, but should be hard to access and mysterious, known mostly from the raiding parties and trade caravans it sends out. Some of the provinces of Atlantis could be spun off as separate factions both geographically and politically distant from the inter-dimensional space slavers.
Although Rifts ultimately traces its mechanics to a fork of AD&D (the Palladium Fantasy RPG), the mechanics are so different that some conversion notes are required as the power curve for Rifts is ridiculously high.
Your first step should always be to see if you can treat a monster as a reskinned standard D&D or AD&D monster. For instance, the splugorth high lord could be an ogre mage / oni, the kydian an ogre, and the blind warrior women as 3rd level monks or fighters. Often the monsters will have bio-wizardry weapons (see below, but basically these are magic items).
MDC should be multiplied by 1/4 to get HP and any creature with MDC should have AC 3 .
For non-MDC creatures, take 1/10 of the sum of HP and SDC. AC should be 7-10 [10-13].
Any attack in MDC should do half that amount in hit points.
You’ll need to have OSR rules for energy weapons, which you can find in Carcass Crawler #2, Hyperborea, Mutant Crawl Classics, or Warriors of the Red Planet. For 5e, try the futuristic weapons entry in DMG. If you don’t have any of these, just treat an energy weapon as a wand of magic missiles, wand of lightning, or perhaps a wand of paralysis (set phasers to stun).
Divide attacks per round by 3 and round down.
Bio-wizardry is a key concept for the setting in which alien creatures are either bound into devices or surgically implanted into someone. When bound to a machine or device, bio-wizardry is grotesque color for magic items but can function as a wand or staff. You may need some game mechanic to keep players from using captured weapons too much, much as drow gear doesn’t work once exposed to sunlight. When bound to a person or monster, it generally is a buff with a cost, such as increasing strength and HP at the expense of charisma.
Tattooed Men get combatant buff spells (e.g., shield, shocking grasp, enlarge person) and summoning spells, but no divination, heavy artillery evocation, or the like. In flavor these spells are tattoos, but they’ll work well with Vancian mechanics. I suggest using elf race-as-class HD, XP, and spell slots.
Pyramids at ley line nexuses are a key concept in the game. Use the artifacts rules from the 1e or 2e DMG, but with the change that they can’t move and lack negative properties for their users.
I would simply cut much of the mecha but to the extent you keep them, consider apparatus of kwalish as a model. Something like the slaver barge can just be a large flying carpet.