The aim of this study is to assess speech privacy in open-plan office using two recently introduced single-number quantities: the spatial decay rate of speech, DL2,S [dB], and the A-weighted sound pressure level of speech at a distance of 4 m, Lp,A,S,4 m [dB]. Open-plan offices were modeled using a DL2,S of 4, 8, and 12 dB, and Lp,A,S,4 m was changed in three steps, from 43 to 57 dB. Auditory experiments were conducted at three locations with source–receiver distances of 8, 16, and 24 m, while background noise level was fixed at 30 dBA. A total of 20 subjects were asked to rate the speech intelligibility and listening difficulty of 240 Korean sentences in such surroundings. The speech intelligibility scores were not affected by DL2,S or Lp,A,S,4 m at a source–receiver distance of 8 m; however, listening difficulty ratings were significantly changed with increasing DL2,S and Lp,A,S,4 m values. At other locations, the influences of DL2,S and Lp,A,S,4 m on speech intelligibility and listening difficulty ratings were significant. It was also found that the speech intelligibility scores and listening difficulty ratings were considerably changed with increasing the distraction distance (rD). Furthermore, listening difficulty is more sensitive to variations in DL2,S and Lp,A,S,4 m than intelligibility scores for sound fields with high speech transmission performances.