The {-o} ending of the ‘hao’ speech style has gone through numerous diachronic changes. The honorific level has been lowered and there has been a functional differentiation between the original form ‘-o/so’ and its phonological variation, ‘-u/su.’ Finally, the limitation to the speaker and hearer rendered the speech style to be rarely used in spoken language these days. This study aims on verifying the sequence, correlation and causes of the various changes to {-o} by focusing on the spoken corpus of the twentieth century.The function of {-o} as a general honorific changed as the aristocracy (the Yangban class) started using the ‘hao’ style mutually. Later, the {-o} functioned as a style through which the speaker casually honors the hearer while not lowering himself/herself. As a result, {-o} lost its original function and became an ending which is difficult to used between a speaker and a hearer who is in a higher position than the speaker. Such usage spread even after the abolition of the status system, as the ‘hao’ style was used mutually between people in a social relationship without any acquaintance, and between strangers. The original usage, however, persevered among women and children who lived only within the local communities.As the honorific level of the ‘hao’ style had been lowered in formal context, vowel rising of the style was not readily accepted. Women and children who were not exposed to formal context and used the ‘hao’ style as a general honorific, however, readily accepted the phonological changes and used it as the ‘-u/su’ form. As a result, the function of ‘-o/so’ and ‘-u/su’ became different.The {-o}, which initially had no limitations to speakers or hearers, became an expression exclusively used by middle-aged or older speakers after the first half of the twentieth century. The spread of ‘hae’ and ‘haeyo’ created a limit to the speakers and hearers, leading to a drop in the usage of ‘hao’ and therefore, ‘hao’ became an ending which is almost never used in colloquial conversations.