I've often thought that there is no need for collusion if you can exchange minimal information over public, legal channels and know how to work a strategy that assumes a modified prisoners dilemma. You can often see oil prices or airline tickets operating that way. E.g when one airline raises prices, then sees that no-one is following the rise, after which the price comes back down. Or everyone else is ready to raise prices and ticket prices go up.

Yup; if judges presiding over anti-trust suits had read Thomas Schelling's book The Strategy of Conflict, there would be many more broken-up oligopolies than there are now.