The ogre-like screams in the middle are Mercury's, and the high harmonies at the end of the chorus hook are sung by Taylor. As the title suggests, it tells the story of a battle between ogres and features a May guitar solo and sound effects to simulate the sound of a battle. The beginning of the song is the end of the song in reverse including the final gong, which when played backward at the start of the song, creates the building wave sound.
The song is one of Queen's heaviest works. The guitar riff along with Taylor's drumming gives it a very "thrash" sound. It was a longtime live favorite. The song was last played on the 1977 News Of The World North America Tour and was performed at every concert up until that tour.
A different version of "Ogre Battle" exists, recorded in December 1973 for the BBC Radio 1 "Sound of the 70s" program. This version starts right away with its riff (without any long intro), does not have any effects that the version on "Queen II" has and sounds much less polished. The BBC version of "Ogre Battle" did originally have a long intro featuring a grand guitar build up; it was not used for this release, allegedly because the original tape was damaged.
The ending gong flows into the next track, with added clock ticks.