explain a little more please,check your oil has no water in it by taking off the filler cap,if there is,then its most probably your head gasket has failed and is letting it in.
Possibly your cylinder head gasket has blown, not an expert but thats what it was when I had same problem on an Escort. It formed a sludgy deposit like coffee cream round the radiator cap that was quite distinctive
on some corsas this can be present but there is no problem with the head gasket, i have a 1.216v version and i get emulsified oil sometimes, its due to short journeys. maybe worth getting checked for peace of mind though
I also have a 1.0, 3cyl Corsa and I had the same problem of emulsified oil around the filler cap. I thought that my Cylinder Head had gone because I was losing water from the radiator but it turned out that I just had a leak and needed a new Jubilee clip.
The sludge around the oil filler cap is caused by lots of short journeys.