I have the exact same thing wrong with my '87 accord 2.0 LX...as soon as you let off the gas just a little it downshifts out of OD, if you keep accelerating it will stay in OD, but as soon as you let off it shifts back down. I'm right at 185k miles

 

When was the last trans fluid drain and fill done?
What is the fluid level when checked per the owners manual?
Inspect the throttle control cable for proper adjustment and movement.