Generalizing of Tarasov method we construct the O(N) Off-Shell Bethe ansatz. In the nesting process a new object, the $pi$-matrix, is introduced to overcome the complexities of the O(N) group structure. par The results we apply to the form factors problem of the O(N) $sigma$-model and the form factors for the essential operators are constructed. The result is compared with usual 1/N expansion in O(N) $sigma$-model and full agreement is found.