After beginning his career with Blue Cross-Blue Shield, Oja started a rental company that specialized in hospital supplies and surgical equipment - the O-J Rental and Sales Company in St. Paul. He was bothered by the poor quality and serviceability of the equipment for injured and invalid people. Many of the medical rental items needed to be modified to accommodate each patient’s needs, so Oja modified them in his workshop. Tiring of constantly modifying equipment to improve its performance, he began building equipment from scratch. He worked out new, innovative designs. Recognition and acceptance of his inventions came early at the University of Minnesota Hospitals where he strove to satisfy hospital and rehabilitation equipment needs. He founded ACTIVEAID, Inc. to manufacture and market his inventions. Oja moved his family and business from St. Paul to Redwood Falls in 1967, and made the shift from retail to manufacturing. He continuously expanded the product line with innovative aids for the relief of pain and suffering in the daily living of invalids and for the care of the injured. The name "Activeaid" has become synonymous with high quality hospital and rehabilitation equipment and particularly the high quality invalid chairs of many types. His inventions have made the company one of the leaders in manufacturing rehabilitation and home health care equipment. The company now manufactures 45 items, 36 are chairs for bathroom use, such as bathing, showering, toileting, and others include motorized parallel bars, bed safety rails, arm counterbalance exercisers, electric tilt tables, supine stands, and emergency rescue boards.

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Oja was actively involved in helping the independent inventor, both by working with and promoting the Minnesota Inventors Congress, a non-profit corporation providing education and assistance to individual inventors, and by working directly with individual inventors.