Optional preferential voting

Optional preferential voting (OPV) is a voting system under which 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 full-preferential voting), 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.

OPV is used in elections in New South Wales and the Northern Territory, Australia.

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. All other Australian lower house elections are use full-preferential voting, where a preference for every candidate must be indicated for the vote to be counted.

In the Victorian Legislative Council, semi-optional voting is used if a voter chooses to vote below the line.

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, Ireland, and Northern Ireland, full preferences are not required.