Hello, Albert -
Clarification: You are asking about an Oster brand Fast-Feed hair clipper. Apparently when you use it on hair cutting clients, the clipper "clips" their skin.
If I am correct in my guess, then . . .
I have found some reviews of the hair clippers which mention the clipper guard combs, which come with the clippers , which may cause problems.
These are links to 2 reviews:
https://www.myhairclippers.com/oster-fast-feed-adjustable-pivot-motor-clipper-review/
and
https://choicebarber.com/oster-fast-feed-review-2017/
The second review includes a recommendation to purchase other clipper guards to use instead of those which come with the hair clipper.
Some of the "clipping" problems may be user error (you), too. You may wish to find and watch some instructive videos. To find such videos, use your favorite search engine (Google, Bing, Yahoo Search, etc.) and the following search phrase, using quotation marks as shown:
"Oster fast feed" demonstration video
Best wishes.