You take a VTT designed primarily for D\&D 5th Edition, find a copy of Into the Odd, and fire it up. You won’t get far, however. You’ll be greeted with spell slot trackers nobody needs, a pack of dusty feats you won’t touch and a character sheet with more empty boxes than the original has attributes.
And that’s the problem in a nutshell.
OSR games are lean, quick and light – a to-hit roll here, a save or two, the occasional reaction if the mood takes you. The software used to support this sort of play needs to be lean and light too. Perhaps the closest you can get is with Foundry VTT thanks to its modules – which allows you to only install what you need. Modules dedicated to games such as OSR virtual tabletop, Knave or Mr Borg support the games for what they are, and don’t attempt to force them into a D&D 5th framework.
Outside of Foundry, however, there are other ways you can still run your OSR games, albeit with a little more player input.
With Roll20, for example, you’re going to need to come up with your own custom character sheets, and craft a number of macros, perhaps alongside a few colourful expletives every now and then as the character sheet stubbornly refuses to play ball. I have definitely done it. Once I spent an entire afternoon trying to get the hex crawl fog of war to co-operate. Despite this, there’s the dark and dirty little secret: very few people play these games for interaction with the software.
They play to test their skill – theirs and the GM’s – as opposed to the mechanics represented on the character sheet. Play it about ref’s being able to read the room, make rulings and improvise from the spirit of the rules – not flick through pages in a rule book whilst frantically scrolling back and forth. There are certainly a number of ways in which you can ensure a virtual tabletop will work, but just as easily it can get in the way if it’s thinking for you.
Plenty of groups forgo such digital frills entirely and play using nothing but voice chat, a dice roller and shared map – which can work wonderfully well for these games.
Less is often more, after all. The strongest argument for any sort of digital interface to be used at the gaming table comes from the inclusion of random tables. Using the Rolltable features in Foundry VTT means a GM can, at the click of a button, bring up an entire table of random roaming monsters, randomly check Morale and randomly discover what treasures the players have unearthed. This process can be incredibly cumbersome on a physical table top, but on the web it’s instantaneous and frankly nobody wants to keep having to cross-reference Appendix B mid combat.