One way is through the rise of a charismatic leader. In an oligarchy, the power is often held by a small group of wealthy or powerful families. These families may be in competition with each other, and this can lead to a power struggle. In the midst of this power struggle, a charismatic leader may emerge who is able to unite the people behind them and challenge the oligarchy. This leader may then use their charisma and popular support to become a tyrant.
Another way that oligarchy can change to tyranny is through the manipulation of the electoral process. In an oligarchy, the voting system may be rigged or controlled by a small group of people. This can lead to a situation where the people are unable to choose their own leaders, and the oligarchy is able to maintain its power. Over time, this can lead to the rise of a tyrant who is able to use the electoral process to legitimize their rule.
Finally, oligarchy can also change to tyranny through the use of force. In an oligarchy, the military or police force may be used to suppress dissent and maintain the oligarchy's power. If the military or police force becomes too powerful, they may be able to overthrow the oligarchy and establish a tyranny.
In conclusion, there are a number of ways that oligarchy can change to tyranny. These include the rise of a charismatic leader, the manipulation of the electoral process, and the use of force.