Kámo

The unusual Czech vocative of kámoš (pal, buddy, friend), kámo, reminds me of Greek declension. It is in Greek where the vocative loses the s (in the case of Czech, š, pronounced sh, close enough) of the nominative. A few Greek examples are patéras, vocative patéra, father; and psarás, vocative psará, fisherman. I wonder why kámo and not the more regular kámoši is used in the vocative. A tip of the hat to Romance languages?