Optional preferential voting (OPV) is a type of preferential voting system under which voters indicate the order of their preferences by numbers. Full-preferential voting requires a preference to be indicated for each candidate. Under OPV, voters may choose to mark a preference for as many candidates as they want. Although complete numbering is not required under OPV, single-preference voters may be required to use a '1' rather than a tick or cross. Some jurisdictions allow ticks or crosses as the voter's intention is clear. Some jurisdictions use semi-optional preferential voting, in which the expression of a minimum number of preferences is required.
OPV is used in elections in New South Wales and the Northern Territory, Australia. It was used in Queensland from 1992 to 2015.
In both the Tasmanian House of Assembly and the Tasmanian Legislative Council, semi-optional voting is used, with a minimum number of preferences required to be expressed; but there is no requirement to complete the entire ballot paper. Elections for all other Australian lower houses use full-preferential voting. In the Victorian Legislative Council, semi-optional voting is used if a voter chooses to vote below the line. Voting above the line requires only a ‘1’ being placed in one box, and group voting tickets voting has applied since 1988.
The Australian Senate voting reform of 2016 switched from full-preferential voting to semi-optional. A minimum number is specified in the instructions on the ballot paper. Since in the past a single number '1' above the line was formal, that is still a valid vote even though voters are encouraged to number six squares.
In other countries, such as Malta, the Republic of Ireland, and Northern Ireland, full preferences are not required.
The ranked-choice voting system used by Maine, USA, can be considered optional-preferential as voters are allowed to rank just one candidate. The system also allows voters to skip one ranking (e.g. marking a first choice and a third choice, but not a second choice). In that case, the next ranking would be advanced to the next highest ranking, but more than one skip exhausts the ballot.