The CONSTANS (CO) and CONSTANSlike (COL) genes play a key role in photoperiod regulation of flowering. In this study, A CO ortholog, OnCOL2, and two COL paralogs (OnCOL8 and OnCOL9) were isolated from Jinhui, a variety of Oncidium, encoding 326, 411 and 291 amino acids, respectively. The bioinformatics analysis showed that they were predictively unstable proteins and located in the nucleus. OnCOL2 and OnCOL8 contained two Bbox domains of Zinc finger and one CCT domain, while OnCOL9 lacked Bbox domain. The multiple sequence alignment and phylogenetics analysis showed that all COLs could be divided into three groups, and OnCOL2, OnCOL8 and OnCOL9 were divided into two different groups. OnCOL2 shared highly identity with Cymbidium ensifolium CeCOL (90.77%). OnCOL8 and OnCOL9 were closer in evolutionary relationship, and exhibited the highest similarity with AtCOL9 and AtCOL10 in Arabidopsis, respectively. They were extremely conservative in the Bbox and CCT domains. Expression analysis showed that OnCOL2, OnCOL8 and OnCOL9 had the highest expression in flowers, roots and pseudobulbs, respectively. Their expressions in leaves showed a periodic trend, and the peak values and time were different under longday and shortday conditions. In the flower bud differentiation stage of Oncidium, their expressions were significantly upregulated in leaves. These results indicated that OnCOL2, OnCOL8 and OnCOL9 were regulated by circadian clock and day length, and their expression upregulated could probably promote the flower bud formation of Oncidium in the photoperiod pathway. The results provide the basis for further study on gene function and photoperiod regulation of flowering mechanism of Oncidium.