So we have these [http://m.ikea.com/gb/en/catalog/products/spr/49805021/chairs]] in white (yes bought before dc) and they get very dirty very quickly. Have been toying with the idea of getting some oilcloth and making a couple of covers, anyone have any idea if this is a good idea or not?
If you are really competent with a sewing machine, then yes, you could do it BUT, oil cloth can be a swine to work with in large quantities... You will not be able to pin it as pins leave holes, you will need bulldog clips, paper clips or clothes pegs. You will need to leave it on a radiator to warm through in order to manipulate it which means it is not amenable to easing.
I would go with a chair slip that you could wash rather than one you could wipe down.
I've covered similar chairs with fabric, so perfectly possible with oilcloth. Although I have only sewn with oilcloth once and found it a little tricky, just sew on the wrong side - other wise it sticks to every thing.