Space opera pretty much begins with A PRINCESS OF MARS by Edgar Rice Burroughs and its various sequels.
I've always considered Burroughs stuff strictly "pulp", even the Barsoom and Amtor stuff. But aspects of these tales certainly influenced space opera.
Later, of course, there was Buck Rogers and Flash Gordon and E.E. "Doc" Smith's Lensmen series.
Buck Rogers first appeared in book form, but IIRC the character and world are pretty different in the book version. He's not even called "Buck" in the book and it's basically "Red Dawn" with America having been conquered by "the Han", a "Yellow Peril" style enemy.
Flash Gordon was King Syndicates answer to the comic version of Buck Rogers, and I'm a quite a bit more familiar with Flash Gordon than Buck. For whatever reason, I don't really associate either with being space opera, though you could certainly make the case that they are.
For me, space opera is the Interstellar Patrol of Edmond Hamilton, the Legion of Space of Jack Williamson and of course.....The Lenmsan series by E.E. "Doc" Smith. I suppose I look at the crazy and vast scale of Hamilton and Smith as one of the defining traits of space opera for me. Weapons that can destroy planets and suns and more. Mighty starships armed with deadly beam weapons, heroic (or roguish in the case of Giles Habibullah) good guys fighting bizarre aliens while rescuing the occasional damsel in distress.
The Lensman series is to space opera what Lord Of The Rings is to fantasy. It pretty much defined space opera. I know serious sci-fi authors tend to dismiss space opera, but it seems to resonate with the general public more than "real" sci-fi. See Star Wars.
Star Trek and Star Wars are the last of the old "super science" epics.