Overt Collusion. Overt collusion occurs when there is a formal agreement (written or verbal) amongst firms to control the market.
Basically the price-fixing agreement is open. Firms A and B could openly collude if there are no competition authorities/laws.
An example is OPEC, there is no international law to stop oil rich Arab countries colluding. A and B cannot openly collude in countries like the UK, US and in the Eurozone because collusion is illegal so A and B cannot draw up legally binding contracts. Alternatively, A and B could verbally agree to price fix and threaten each other with a credible threat to deter cheating.