Hiya peaches, sometimes even a knowledgable source can make a typo or an error and not catch it before printing, Chicken Teeth's idea about the sump sounds good as a solution until you get a bigger tank. I know they are a bit hard to find but a 50g tall or a 50g long is a better choice than the 55g standard, they are 36"x 18"x 17" and 48"x 18"x 13" instead of the 55g's 48"x 12 1/2"x 21", the extra 5 1/2" in width is good for larger fish like your oscar.
HOB filters work fine though they have less room for biomedia than do canisters, I use more than one on larger tanks and like the gph to be 3-4 times the tank capacity. I remove the charcoal from the filter pack and replace it with either filter batting or small pieces of lava rock for a little more growth space for the bacteria. Using two or more HOBs also lets me alternate which one I clean so it helps keep the water parameters stable.
HOB filters work fine though they have less room for biomedia than do canisters, I use more than one on larger tanks and like the gph to be 3-4 times the tank capacity. I remove the charcoal from the filter pack and replace it with either filter batting or small pieces of lava rock for a little more growth space for the bacteria. Using two or more HOBs also lets me alternate which one I clean so it helps keep the water parameters stable.