There is a new option in the OCAD Preferences to use the mouse wheel to zoom in and out instead of scrolling.

Use mouse wheel to zoom in and out instead of scrolling is unchecked (default settings):
Scroll Mouse Wheel = Scroll map vertically
Scroll Mouse Wheel + CTRL = Zoom in and out
Scroll Mouse Wheel + SHIFT = Scroll map horizontally

Use mouse wheel to zoom in and out instead of scrolling is active:
Scroll Mouse Wheel = Zoom in and out
Scroll Mouse Wheel + CTRL = Scroll map vertically
Scroll Mouse Wheel + CTRL +SHIFT = Scroll map horizontally