I really like the OP-1 synths - at first I thought they were a let down, now I really like them, especially with chords. Most of them have a nice fuzzy, (almost lo-fi) digital character that just works for so many sounds.
Im looking for the same kind of digital sound on the computer or ipad. The closest I have found have been some ensembles in Reaktor, particularly for Dr Wave kind of lo fi wavetable. However generally the usual VA, Wavetable and FM synths don’t sound the same as the OP-1. Most of my VSTS’s tend to sound like ‘vsts’ sadly.
Most plugins won't sound close to the OP-1, because they're aiming for a very high fidelity sound.
Plogue Chipsounds emulates nearly every popular video game sound chip of the 80s and early 90s. There are a lot of free plugins for that sort of thing, too. Search for things like 'NES VST' or 'lofi wavetable plugin' and you'll find a lot of stuff.
Free 8-bit shaper from xfer records:
http://xferrecords.com/freeware/
Bitwiz on iOS is great, I can't remember if it was free or not, but it's certainly fun once you get into it.
I also highly recommend Chipsounds, it actually is sample based, and the sounds are very hi-fi, yet lo-fi at the same time. It’s the only VST I’ve ever not regretted buying and I keep it around for when I play around ITB.