A 20 mm lens has an 84 degree horizontal angle of view. Verticals is less. And, this is true only when attached to a standard 36 x 24 mm sensor (the standard frame size of 35 mm film.). On a smaller sensor, such as the APS size, the field of vies is smaller.
Most, but not all, DSLR cameras have the APS-C sized sensor or DX sensor, which is around 23-25 mm wide and approximately 16 mm tall. This dates to earlier times when it was very difficult, and expensive, to manufacture full frame 36x24 sensors.
Olympus has or had an even smaller sensor in their lineup, approximately half the dimensions of the standard 35 mm film-based “full frame” size.
Good point. It was a Canon T3 Rebel according to the OP, which I think is a APS sensor camera, not a full frame one.
So the effective angle would be like a 30mm. Still pretty wide.