Yes, but, with the Oppo 981 you will be scaling twice: 480i->720p->~800p, or 480i->1080i->800p. That will lose you resolution and introduce artefacts. The sad news is that you just don't need another scaler (or, indeed, another DVD player). Just set your Coby to 480i output (if it can do that) and let the TV do all the scaling. Don't let the Coby do the deinterlacing.
The difference between the Oppo 981 and 980 came down to the fact that only the 980 has 480i output over HDMI. And 480i is slightly smoother on pans than 480p, presumably because allowing the TV to do both deinterlacing and scaling in one operation works better.
Try it for yourself (and let us all know) but I believe you will find that you get no picture improvement using the Oppo to scale to 1080i (or 1080p). Look at smoothness of motion, etc. as well as "sharpness" (which is often just fake digital enhancement in the DVD player).