A few firms who cannot look away
So far on this rung you have met two extremes. In perfect competition there are so many tiny firms that each is a price-taker — no single seller can budge the market, so nobody watches anybody. At the other end sits the monopoly, one firm alone, with no rival to watch at all. [[oligopoly|Oligopoly]] lives in the uneasy middle: a *few* large firms, each big enough to matter, each holding real market power, yet none big enough to ignore the others. Think of the handful of firms behind your phone's operating system, the planes you fly, the soft drinks on the shelf, the search engines you actually use.
Why do markets end up with just a few giants rather than dozens? The usual answer is the same barriers to entry you met in the monopoly guide, only partial: huge fixed costs and economies of scale that reward bigness, patents and networks, control of a key input, or a brand built over decades. These barriers are high enough to keep the crowd out, but not so absolute that they leave a single survivor. The result is a small club — and in a small club, what each member does lands squarely on the others.
Interdependence: the move that depends on their move
The defining feature of oligopoly has its own name: [[interdependence|interdependence]]. In every market you have studied so far, a firm could draw its demand curve and choose its best output without a thought for what any single competitor would do. Here that is impossible. If you cut your price to win customers, your three rivals feel it immediately — and they will respond. So your best price depends on their reaction, which depends on their guess about your reaction, which depends on your guess about their guess. The decision folds back on itself.
This is exactly why oligopoly is the bridge to [[game-theory|game theory]] — the topic that fills the next rung. A monopolist solves a calculus problem against passive customers; an oligopolist plays a game against thinking, reacting opponents. The right question is no longer "what output maximises my profit?" but "what is my best response to what I expect them to do, given that they are asking the same about me?" Strategy, not just optimisation, takes over. Hold that thought: it is the single idea that makes this market different from everything before it.
Cartels: the temptation to join hands
If watching each other is exhausting and price wars are ruinous, why not just agree? When a group of firms openly or secretly coordinate — fixing prices, carving up the market, or capping output — to act as one, they form a [[cartel-and-collusion|cartel]], and the cooperation itself is called collusion. The prize is enormous: if a few firms behave as a single monopolist, they can together restrict output, push the price up, and split monopoly-sized profits. The most famous example is OPEC, the club of oil-exporting nations that meets to set production quotas. Collusion is, in effect, a private treaty to stop competing.
Here is the catch, and it is beautiful. The very deal that enriches the cartel also tempts every member to break it. Suppose the cartel agrees to keep the price high by holding output low. Each firm now stares at a high price — and realises that if *it alone* quietly sells a bit extra at that lovely price while everyone else keeps their promise, it pockets a fortune. But every member sees the same tempting gap. So each is pulled to cheat, and once enough do, the flood of extra output pushes the price back down. The cartel devours itself from the inside. This is why collusion is chronically unstable — not because firms are honest, but because the incentive to defect is strongest precisely when cooperation is working best.
This tug-of-war between the gain from cooperating and the temptation to betray is the textbook [[prisoners-dilemma|prisoner's dilemma]], the most famous setup in game theory, which you will meet properly on the next rung. The cartel is simply that dilemma played by firms: collective restraint would make everyone richer, yet each player's private best move is to cheat — so the cooperative outcome keeps falling apart. Add that openly fixing prices is illegal in most countries (that is what antitrust law is built to catch), and the wonder is not that cartels collapse but that any survive at all.
The kinked demand curve: one story of sticky prices
Oligopoly prices are often strangely *sticky* — they sit unchanged for long stretches even as costs wobble. One classic story for this is the [[kinked-demand-curve|kinked demand curve]]. Picture an oligopolist deciding whether to change its price, and make one assumption about how rivals react. If you *raise* your price, you bet your rivals will gleefully sit still and let your customers stream over to them — so you lose a lot of sales, meaning demand above today's price is very elastic (sensitive). But if you *cut* your price, you bet your rivals will match you instantly to protect their own sales — so you gain almost nobody, meaning demand below today's price is very inelastic.
Splice those two halves together and the demand curve each firm faces has a sharp *kink* at the going price — flat and elastic above it, steep and inelastic below it. The upshot: raising price loses customers fast, and cutting it gains almost nothing, so neither move pays. The price gets trapped at the kink. The model even has a clever wrinkle: at the kink the marginal-revenue curve jumps with a vertical gap, so a firm's marginal revenue can shift quite a bit — costs can rise or fall within that gap — and the profit-maximising price still does not budge. That is sticky pricing falling straight out of interdependence.
Above the going price P*: raise price -> rivals DON'T follow
-> you lose lots of sales (ELASTIC, flat)
Below the going price P*: cut price -> rivals DO follow
-> you gain few sales (INELASTIC, steep)
Price
| \ elastic (rivals hold)
| \
P* |----o <-- KINK: price sticks here
| \____
| \___ inelastic (rivals match)
+---------------- QuantityWhy oligopoly is the hardest market to model
Notice what just happened: to even draw that kinked curve we had to *assume* how rivals would react. Change the assumption and you get a different curve, a different answer, a different model entirely. That is the deep reason oligopoly is the hardest market structure to pin down. Perfect competition has one clean prediction; monopoly has one clean prediction; oligopoly has a whole zoo of models — kinked demand, price-leadership, firms competing on quantity, firms competing on price, repeated games where reputations matter — and which one fits depends on details no theory can settle from the armchair.
The honest takeaway is that oligopoly has no single tidy diagram, and that is not a failure of economics — it is a faithful reflection of a world where a handful of clever, watchful players each try to outguess the others. The discipline that does the best job here is the game theory you are about to climb into: it does not promise one prediction, but it gives a precise language for strategy, threats, promises, and the conditions under which cooperation can actually survive. Oligopoly is where the static curves of supply and demand finally hand the microphone to strategy.