Well, they vary a lot because they're all from different parts in an 80 year span.
Let’s not forget that Ornstein started out as a "futurist" composer, practically inventing the tone cluster (though Cowell cites Ives), and achieved tons of fame, having a biography written about him in 1918 (84 years before he died!). Following that 5 year period (about 1915-1920) when Ornstein was the poster child for modernism he essentially abandoned modernism and performance and sorta faded into obscurity.
All of the Ultra-Modernists essentially disowned him, as did Schoenberg (whom a critic apparently said Ornstein made look "tame"), etc.
I mean, he has some tonal works that he wrote at the same time as Suicide in an Airplane and some more "modernist" works later on, but for the most part, he started out as a "futurist" composer and then went the (neo-)Impressionist route.
When he essentially retired from performing and switched styles no one paid any attention to him.
But, even the tonal works are varied quite a bit. That's most likely due to the fact that he was 108 when he died (though his last work was when he was 97) -- 1893-2002 (he lived in 3 different centuries!) -- that’s a lot of time to develop your style.
But the problem with Ornstein is that there isn't really anything novel about his work after Impressions of the Thames, Wild Men's Dance, or the aforementioned Suicide on an Airplane. Yes, he produced some nice works, but nothing that stands out too much against his contemporaries (at least in my opinion). That and the fact that American music in general is still an ignored topic in the music world -- up until the 20th century Europeans didn't think of it as being on the same level as their music (that attitude still sticks for pre-20th century music, I mean, how often do you hear of MacDowell, Beach, or Parker -- yes, Ives was essentially the first major American composer not trained in Europe, but still) and unless you did something major in Europe (Cage and Feldman at Darmstadt, for example), you're pretty much under the radar to most historians and musicians (and, more importantly, conservatories and schools), Cowell, for example, is a known composer, however Stockhausen is credited with being the first to link rhythm and form to pitch via the harmonic series (I've read articles that say exactly that), even though Cowell wrote about it in New Musical Resources in 1919 (keep in mind that Stockhausen wasn't born until 1928) -- is the main reason though, that you're now posting about Ornstein in the "holy moley I can't believe I've never heard of this guy" kinda way. I mean, Ornstein is completely forgotten now; Cowell typically gets credit for the cluster, sometimes Ives. Ornstein's contributions mostly go ignored.
That all being said, I'd say that (though it may seem varied) Ornstein was never not an Impressionist composer.
Sorry about that little rant mid sentence there, I've studied American music pretty closely and get pretty excited and overenthusiastic about it.
P.S. - Scores for all of Ornstein's work are available for free (legally) here.
P.P.S. - Nice picture of Leo.
P.P.P.S. - Sorry about refering to the United States as "America", or at least only refering to US composers -- there are pleanty of examples of perfectly great composers from, say, Brazil composing at the time of Mozart but whom don't get any mention. The Americas as a whole are a neglected topic in "Western" music.