The O(2) model in Euclidean space-time is the zero-gauge-coupling limit of the compact scalar quantum electrodynamics. We obtain a dual representation of it called the charge representation. We study the quantum phase transition in the charge representation with a truncation to "spin S", where the quantum numbers have an absolute value less or equal to S. The charge representation preserves the gapless-to-gapped phase transition even for the smallest spin truncation S = 1. The phase transition for S = 1 is an infinite-order Gaussian transition with the same critical exponents δ and η as the Berezinskii-Kosterlitz-Thouless (BKT) transition, while there are true BKT transitions for S ≥ 2. The essential singularity in the correlation length for S = 1 is different from that for S ≥ 2. The exponential convergence of the phase transition point is studied in both Lagrangian and Hamiltonian formulations. We discuss the effects of replacing the truncated Û ± = exp(±i θ) operators by the spin ladder operators Ŝ± in the Hamiltonian. The marginal operators vanish at the Gaussian transition point for S = 1, which allows us to extract the η exponent with high accuracy.