The 16-year-old joined United from Arsenal earlier this month and made his debut as a substitute last week.
Today, Obi-Martin made a first start for his new club in their 6-0 win against Nottingham Forest.
And it was the centre-forward who dominated the scoresheet with a 14-minute hat-trick.
His first goal came from capitalising on a Forest defensive error after just 20 seconds.
He then scored from the edge of Forest's penalty area on five minutes and completed his treble in the 14th minute with a first-time finish from close range.