This contribution deals with the declension of Czech pluralia tantum oikonyms ending with -y in written Czech; the research was based on the SYN2005 corpus. The research focuses on two areas - firstly, feminine oikonyms with regular declension, and secondly, oikonyms with irregular declension. The corpus material contains 108 oikonyms ending with -y; there are 82 feminine oikonyms with regular declension and 26 irregular oikonyms. The oikonyms found in the corpus represent mostly the names of villages, towns and neighbourhoods. Irregular oikonyms can bear variant endings or doublet endings in Gpl., Dpl., Lpl. and Ipl. Tendencies and factors influencing the choice of endings in Lpl. can be objectively determined from the SYN2005 corpus material; there is a growing tendency to use the feminine variant ending with -|ch instead of the doublet -|ch/-ích. Dpl. and Ipl. forms are represented very rarely in the corpus, however, it can be said that written usage documents the peripheral occurrence of feminine endings (-|m, -ami) alongside the masculine endings (-ům, -ech, -ům/-ech; -y) which commonly occur in these two cases. Occurrence of the endings -ů, -Ø in Gpl. is not caused by intrusion of feminine endings into the declension of masculine oikonyms, since both these endings are masculine. The majority of irregular oikonyms in the corpus has the original zero ending; only some names ending with -ky use the doublet ending -ů/-Ø.