Honestly? There's no clean Dulux match for Olive Colour, and I'd be doing you a disservice to pretend otherwise.
The closest Dulux gets is Olive Grove (LRV 8.1), and it comes in at ΔE 5.6 from Little Greene's Olive Colour. Anything under 2.5 reads as "very close" — under 1 is imperceptible — so 5.6 is a match you'll clock the moment you put the two boards side by side. It's in the same family, a deep olive, but the undertone and depth drift off the original.
The other contender is Tarragon Glory 1 (LRV 6.2), and that's further still at ΔE 7.5 — darker and muddier than Olive Colour. I wouldn't reach for it unless you specifically want a heavier, more enclosing green.
Here's the thing about Olive Colour: it's a properly characterful, slightly dusky historic olive, and that nuance is exactly what gets lost in translation. Cheaper olives go either too yellow-khaki or too grey-army, and Olive Grove leans a touch flatter than the Little Greene. On a north-facing wall the difference will be even more obvious — cooler light pulls the grey out and flattens olives, so a 5.6 gap becomes very visible.
My honest advice: if you're set on Olive Colour, buy Little Greene Olive Colour. It's a low-LRV shade so you'll want a properly pigmented paint to hold its richness anyway, and the Intelligent or Absolute Matt finishes deliver that depth in a way a substitute won't. The few quid you'd save going Dulux isn't worth losing the colour you actually wanted.
If budget genuinely forces the Dulux route, go Olive Grove as the lesser of two compromises — and order a sample pot, paint two coats on lining paper, and live with it on the actual wall for 48 hours before you commit. Olives are unforgiving and they shift a lot through the day.