Can an small abnormal ovarian cyst cause a false positive ovulation test? first day of positive opk . regular 28day cycle.chart shows ovulation as wel

Probably: Depends on what the definition of "small abnormal cyst" is? But any follicle that's functional can trigger an lh surge to start the ovulation process. The other possibility if you were scanned at a particular cycle day, is that you ovulated before the scan even as close as minutes before the scan and the early corpus luteum could be missed leaving only the "small cyst" to be seen on ultrasound.