As above, start with a steam cleaner. Sorry Aiden but a wire brush won’t do s**t for oil contamination.
Grinding the top off afterwards will help a Lot and is probably worth it just for giving you a smoother surface. Small voids can be filled with a resin repair compound or a mix of 1:1 kiln dried sand and cement with sbr as the sole wetting additive. If a large area then I’d hire a proper grinder, but a 9” with a diamond grinding cup is good for smaller areas and 5” for corners etc.