High Hopes Hailla Ball
The liner notes for Led Zeppelin's album Houses of the Holy contain the lyrics for all of the songs therein, and The Ocean is clearly shown to contain the line "Got a date, can't be late for the high hopes hailla ball." Even so, the last four words of that line bear scant resemblance to what's in the actual song, and many different theories have been sugested about what Robert Plant sings, usually revolving around the somewhat specious suggestion of "hellhound", as in the famous Robert Johnson song Hellhound on my Trail.
Unfortunately, there's no mention as to whether the lyrics are directly from the artist or transcribed by the record company, but they're definitely unreliable in other places too. They say, for example, that the line before the one in question starts with "ain't" whereas in actual fact it starts with "got", and even the beginning of the line in question itself goes "I got a date, I can't be late", so that's different too.
On a site devoted to Song Meanings, one commenter has noted that the lyrics seem to be "hell high hella ball" in the version on How The West Was Won, which is actually how I'd transcribe the Houses of the Holy version were it not for the liner notes. Jeff Lybarger, too, agrees that the album version seems to start with "hell high", and both the How The West Was Won and Led Zeppelin DVD versions also sound like they start with "hell high", though more interestingly it seems that the third word in both of those versions ends with an "s". Only one site contained the transcription "hell high haillas ball", and even that's now down, preserved only in the Web Archive.
Googling for the word "hailla" brings up only 1,320 results, and about half of those refer to the song with the rest being names of things or otherwise unrelated. On the other hand, the word "heilla" brings up 16,300 results, mainly Icelandic, and some of those are pretty interesting. I doubt this is related, but in a page on Old Norse it says that the word "holy" is related to the word "health", and moreover to the Old Norse word "heilla", which means "to invoke spirits".
It may just be that the syllables are space fillers, an inside joke, or something like a placename or actual event that therefore has a slightly weird pronunciation, but until Robert Plant himself clears the mystery up I don't think we'll find out for sure.
Cite: Palmer, S.B. (2005). "High Hopes Hailla Ball", in: What Planet is This?
Archival URI: http://inamidst.com/notes/hailla