your JD and Oscar will not breed . . . they are not related in any significant way, other than both being cichlids . . . as noted, they come from very different regions and have very different genetics . . . no sugar-coating, it is not going to happen
the reason your 55G will be too small for a full grown Oscar is that the width of a 55G tank is only approx 12" . . . a lot of fishkeepers (myself included) would like to see the width of the tank be more like 1.5 to 2x the length of the adult fish . . . so a 10-12" adult Oscar would be much better in a tank that is 18" wide, like a 75G or 90G . . . 24" wide would be better still
a female JD might typically max out at around 8", so a 55G is not a bad size for the JD, but it would be too small for the oscar
bottom-line, this is just advice, it's up to you whether or not to follow it . . .