OEC candidates are eligible for unavailed seats under SC/CT quota.What is this unavailed seats.How to get them?Are these seats available only after 1st round allotment?

Un-availed seats are the seats that could not be filled even after all the counselling rounds. Generally quite a few reserved category seats go un availed (depending on the exam) as the number of students appearing under that category are less or simply enough number of reserved category candidates could not clear the cut off scores. When such a thing happens those seats are converted into other category seats in order to accommodate more students and ensure that the seats don't go unfilled. To apply for those seats you need to be constantly up to date regarding any notifications released by the universities (usually on their official sites) these notifications are out for only short durations and those seats get filled quickly.