When dispensing oil from a tin can, why do we usually make TWO holes in the tin can?
To allow ingress of air AS WELL as displacement of the liquid.
Back in the day, before the advent of ring pull cans, you used to have to make 2 holes in a can of soft drink with a tin opener: one to drink from, and the other hole to allow intake of air to displace the liquid inside. If you only made the one hole, the exit of liquid was interrupted: some liquid would flow out, it would stop, and the flow would start again briefly.
If you have an empty jam jar with a metal lid, you can do the experiment yourself. Make one hole in the lid, see how water flows out, and then make another hole, and exit will be easy. Of course, for a viscous liquid such as oil, the 2 holes will be even more important to promote even flow.