The N—H...O hydrogen bond is the characteristic interaction in the crystal structures of N-benzyl-P-phenyl-N′-(p-tolyl)phosphonic diamide, C₂₀H₂₁N₂OP or (C₆H₅)P(O)(NHCH₂C₆H₅)(NHC₆H₄-p-CH₃), (I), diphenylphosphinic 1-methylpropylamide, C₁₆H₂₀NOP or (C₆H₅)₂P(O)[NHCH(CH₃)(C₂H₅)], (II), (S)-1-phenylethylammonium N-[(S)-1-phenylethyl]phenylphosphonamidate, C₈H₁₂N⁺·C₁₄H₁₅NO₂P⁻ or [S-(C₆H₅)CH(CH₃)NH₃][(C₆H₅)P(O){S-NHCH(CH₃)(C₆H₅)}(O)], (III), and (4-methylbenzyl)ammonium diphenylphosphinate, C₈H₁₂N⁺·C₁₂H₁₀O₂P⁻ or [4-CH₃-C₆H₄CH₂NH₃][(C₆H₅)₂P(O)(O)], (IV). This article focuses on the N—H...O hydrogen bonds by considering the structures of (I), (II), (III) and (IV), and reviewing their analogous compounds, including 43 (C)P(O)(N)₂, 102 (C)₂P(O)(N), 31 (C)P(O)(N)(O) and 96 (C)₂P(O)(O) structures, deposited in the Cambridge Structural Database (CSD). For the structures with a (C)P(O)(N)₂ segment, only neutral hydrogen bonds were found in the CSD. The other three classes of compounds included both neutral and `charge-assisted' hydrogen bonds, and the (C)₂P(O)(O) structures were particularly noticeable for a high number of cation–anion compounds. The overall tendencies of N...O distances in neutral and cation–anion compounds were compared. The N—H...O hydrogen-bond angles were also analyzed for the four classes of phosphorus compounds.