55G is the absolute minimum for a single oscar, and even that is only generally "accepted" rather than recommended - ie it's considered to not be enough, but as good as a lot of people are going to get. Anything else in here is liable to end up as a bloodbath. The water volume is survivable, the swimming space is poor. An Oscar will take the entirety of this as its territory.
75G is the real minimum for a single Oscar, as it has enough space for swimming and enough water volume. Although "enough" is the operative word - it's at the limits of acceptable. It does not have enough "footprint" for territories to form, though, so more fish could potentially be a bloodbath. Ie it doesn't have space for two Oscars to both have a territory, or even an Oscar and a non-territorial fish (the other fish can't stay out of the Oscar's territory). People do sometimes manage to put two Oscar's in a 75G, but they MUST be introduced young, and even then you're likely to end up with problems eventually.
100G is a minimum for an Oscar with any sort of tankmate. Either an oscar or a couple of smaller-but-not-too-small cichlids. Even here you could get some problems because there isn't a big enough footprint for two Oscars to have a proper territory, so it depends on your fish and their temperament. A 100G is normally enough for an Oscar and some smaller, less aggressive fish, though - Silver Dollars, Severum etc... the Oscar takes most of the territory, but there is enough space for the others to stay out of the way. You can get away with two Oscars in a 100G if you are lucky with their temperament and they're introduced young.
125G is the real minimum for two Oscars, as it gives them enough space to have a territory each. Even so, you should introduce them small and be prepared to move one if they take a disliking to each other. This is also enough for an Oscar and a handful of tankmates
I keep my Oscar in a 110G with some Severum (4, from large to small). I'm somewhere between the 100G and 125G, so there's a chance I might end up overstocked when my fish are all bigger: when dealing with large cichlids, though, you always have to be prepared to move some fish out when necessary. Mine will likely be moved to a 150G before this is an issue anyway, but if not then I may have to reduce the number of Severum.
IMO, a 100-120G with a Single Oscar and a few Severum gives a very nice tank.
Oscars will destroy and eat plants. They'll destroy most things, too - most fancy themselves as interior designers, but lacking opposable thumbs they're much better at knocking things over than putting them back.
DO NOT put them in with any fish which fits in their mouth. It WILL be eaten. Not might be eaten, will be eaten.
