is this good enough for a oscar?

Absolutely not, I'd say the MINIMUM size for a SINGLE oscar is 55 gallons with little to no decor. but even then only an average sized oscar has his length in turning space. Also oscars are waste machines, I own a 13" oscar in my 120 and I have a gph rate of 1083, sure I have other fish but this is easily the messiest fish I own. Also you'd only be able to keep an oscar in a 40 until it's 8" MAX.

No, a small under filtered tank does not make for healthy fish,

so no , no experienced person would say a 40 or a 75 was big enough,, maybe for a year at most.

and nobody would say that filter was adequate (I'd go with about 320gph in a 40)

besides, aqueon is a crap brand even if you had a bigger one, just doesn't have effective biologic filtration.

Yes doing water changes would help at first, but eventually the size of the tank will affect it's health.

Note: and by the way, algae isn't harmful in any way, so how often you plan on cleaning the glass isn't important to this discussion

No Oscars are very very gross fish much like the common gold fish. Any undersized filtration does not make a healthy livable aquarium for any fish. It would not help to vacuum because then the right bacteria for the fish to live will not be stable and can cause even more problems. You will have to eventually upgrade your aquarium for the Oscar to thrive. I also suggest a Fru-val filtration system.

You should care for a goldfish they are very interactive than oscars, they need only 5 gal for fancy goldfish but have a powerful filter.

Most all fish can survive in any size tank for a time -- The bigger it is makes us happier not the fish

NOPE