During routine OM&M of an oxygen diffusion system, the Stratus Technician found a black widow in a component he was holding.  The Technician was conducting a routine change-out of an oxygen cylinder.  The new cylinder had been mounted to the safety rack and the valve protection cap was removed.  Based on the Technicians experience, he was aware that these caps are a hiding place for Black Widow spiders and during the removal stayed aware of his glove and shirt sleeve to make sure a spider did not leave the cap and climb to his glove or shirt sleeve.  The inspection of the interior of the cap is not possible while it is installed on the cylinder.  Immediately after removing the cap, the Technician inspected the inside of the cap and discovered there was a spider inside.  The Technician immediately tapped the cap on the ground to eject the spider and stepped on the spider to kill it.  After it was killed, the technician identified the spider as a Black Widow.  The Technician was wearing leather gloves and a long-sleeved shirt and was cognizant of spiders in the protective caps therefore minimizing to possibility of being bitten.