The Officers ACP has been out of production for a good while. The Defender is it's replacement and a big improvement in my opinion. The OACP was mostly made in carbon steel in .45 ACP. It had a few design flaws (again, my opinion) that were done to get the gun chopped down that short and still get around a bunch of patents that Detonics had active. The method of retaining the recoil spring plug in the slide was a pretty delicate little tab of steel sticking out the side of the plug that engaged a slot in the rear end of the slide's spring tunnel. The tabs would occasionally break off, launching the plug down range. The barrel was a tapered bull barrel configuration that still used a barrel bushing, albeit a considerably larger one that the slide was bored out to accept. Of course, this was all designed and built in the hey-day of Colt's sorriest quality control and loosest tolerances, which all just made everything worse.
The Defender has a bull barrel with no bushing and a two-element captive recoil spring assembly. It is engineered to allow maximum rearward travel to the slide which enhances extraction and ejection. Tolerances are better as are materials. All around a better mini .45.