What is oligo-amenorrhea?

Too few periods. Oligo- means few (or little); menorrhea describes menses, or periods. So oligomenorrhea means having too few periods, usually defined as 6 or fewer in 1 year. A- means no, so amenorrhea means no periods.
Less or No periods. Oligo implies less frequent periods and women who do have a period are said to have amenorrhoea.