I presume we're talking about the engine oil level light... if so, first make sure you indeed do have an engine filled to the right level with oil!!
If it is the engine oil level sensor that's at fault, it's not the big nut - it's the triangular flange held on with 3 x bolts which need a 8mm spanner (I think). You'll need a new gasket however that's about it - it's simply a case of of removing the unserviceable item and replacing it. Don't forget to drain the oil first! Good opportunity to change the filter as well - if you've never worked on them before, you'll be cursing when you find how fiddly and messy that bit is!
However, can I caveat all that - this applies to the turbo engine and I presume that it's the same on NA.
