Oil warning light permanently on

Hello, my oil warning light has been permanently on since having two turbo hoses replaced. Since then all has been perfect, but the light is permanently on. The oil level is consistently fine when checked. When it needed servicing, the service light came on and I then had full oil and filter change and the ‘service’ light went off (or mechanic turned it off?), but the oil warning light stayed on.
Any advice?
I’ve gotten used to it and always been happy to check my oil level regularly, but now I want to sell the van i would ideally like to solve the problem for the buyers peace of mind! Thanks in Advance. Fio.