Off-Black is a brilliant colour — a proper soft, greeny-charcoal black with none of the flat harshness you get from a true black. But you don't have to pay F&B money to land in the same place.
The nearest match is Dulux Charcoal Drift (LRV 7.6) at ΔE 2.5 from the original. Anything under 2.5 is what we'd call very close — most people couldn't tell them apart on a wall, especially once it's dry and living in real light. Dulux Heritage in particular gives you a lovely chalky finish for a fraction of the cost.
Close behind is Crown Anthracite (LRV 7.3) at ΔE 2.7, and COAT The Record Store (LRV 5.2) at ΔE 2.8. The COAT option sits a touch darker and deeper — that lower LRV means it reads as a fuller black, so if you actually want it a shade richer than Off-Black, that's your one. COAT also do quick-ship sample pots and the paint's genuinely good quality, eco-credentials and all.
Now the but what about bit: Off-Black's magic is partly its undertone — that subtle warmth that stops it going cold and morgue-like. No dupe is going to be a perfect carbon copy of that undertone, which is why I'd always say buy testers before committing. Paint two coats on a big bit of lining paper and move it around the room over a day. North-facing rooms will pull all of these cooler and greyer; south light warms them up nicely.
For woodwork, all three take an eggshell or satin happily. If you're doing a feature wall or panelling, the matt/estate-style finishes hide imperfections better than anything shiny.
My honest pick: go Charcoal Drift if you want the closest like-for-like saving, or The Record Store if you want a deep dramatic black with cracking paint quality. Either way, sample first — black is unforgiving if you guess.