Optional Preferential Voting (OPV) is a system of vote-casting.
Under OPV, voters may choose to mark a preference for one candidate (effectively voting as though it were a first-past-the-post election), all candidates or any number of candidates in between. Although complete numbering is not required under the OPV system (effectively a full-preference vote), single-preference voters are supposed to use a '1' rather than a tick or cross. In practice, ticks or crosses are still considered valid votes as the intention is clear.
OPV is used in New South Wales and Northern Territory elections in the Commonwealth of Australia.
In both the Tasmanian House of Assembly and the Tasmanian Legislative Council, semi-optional voting is used - a minimum number of preferences must be expressed, but there is no requirement to complete the entire ballot paper. All other Australian lower house elections are run under full-preferential voting, where all of the candidates must be ranked in order of the preference of the voter to be counted.
In the Victorian Legislative Council, semi-optional voting is used if a voter chooses to vote below the line.
In 2016, Senate Voting Reform in Australia switched the Senate ballot papers from full-preference 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, Ireland, and Northern Ireland, full preferences are not required.