An Odd Pair Sudoku puzzle is based on a a standard Sudoku grid but with the addition of 'o' "odd pair" markers between certain squares, 'o' standing for "odd". These indicate that the pair of squares sum to an odd number (such as 3 and 6 for a total of 9, an odd number).
The rules of Odd Pair Sudoku mean in practice that one of the numbers in each pair must be odd and the other must be even. There is no 'inverse rule' however - the absence of an odd pair marker between two squares has no meaning, and the pair of squares can be either odd or even in total value.
As with all Sudoku puzzles, there is a unique solution which can be reached via reasonable logical deduction.