a partnership is when 2 or more people start a business together and split the cost liability and revenue
A sole proprietorship in a competitive industry competes with others with the same exact product and only competes the price and a monopoly in a competitive industry creates one product and has no competition
A oligopoly is a market structure in which a small number of interdependent firms compete
For example phone companies are oligopolies. They pretty much all do the same just with different terms and conditions.
In order for them to be a partnership they have to be part of the company or multiply companies. Since its going to be a partnership the people that are involved have to split the cost liability revenue
Hopefully now you have a better understanding of partnerships and oligopolies