There's no empty spaces left behind after oil extraction.
The oil is in microscopic gaps in the rock. The grains of material that make up the rock are irregularly shaped, which means there gaps inbetween them called pores. Other "gaps' might be fractures or fissures.
The pores and fractures connect to make a network, so the water can flow through the rock.
In general, these gaps are filled with water; at depth this is brackish and undrinkable. When oil is generated underground its is less dense than water, so it floats up through the pores and fissures.
Some oil gets to the surface, but some might get stuck below a rock layer that doesn't have small pores or fissures - like a clay, for example.
This trapped oil is like a bubble. The drillbit goes through the "trapping" layer, into the bubble, and the oil can flow out into the borehole and up the surface.
As the bubble shrinks, the water pushes back into the spaces where it was.
There can be some subsidence as Christian Bergland comments, but that is more because of the change in pressure, as opposed to the oil being removed. This usually only happens when a very large oil or gas field is produced for many years.
TLDR; There's no gaps left behind, water flows in to replace the oil. The change in pressure can lead to some subsidence over a long period of time.
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