Was "oop" really more common than "oops" till 1990?

Google Ngrams shows a marked preference for oop over oops up until 1990:

Is Ngrams to be trusted here? Is it strange that I've never seen oop in writing? Even Dictionary.com doesn't have anything more than acronyms under oop.

Solution 1:

No.

Click under the graph to get the actual hits. You find

But I feex you oop goot.
And oop there yonder in those trees,
dominant faces being ∞p (crystallography; this oo is really infinity),
Soo-oop of the ee-evening. Beautiful, beautiful Soup!

as well as a number of other "oop"s, including the call of the sooty grouse. But I didn't see any instances of "oop! I made a mistake." Most of the "oop"s seem to be "up"s spoken with a pronounced accent.

Solution 2:

EtymologyOnline says oops is only attested from 1933.

To me it is likely to be a shortening of oops-a-daisy. The Phrase Finder tracks that back to upsa daesy in "The dialect of Leeds and its neighbourhood" in 1862, to up a-dazy from Jonathan Swift in 1711 and to upaday even earlier.

If oop is a dialect form of up, and oops of ups, then you might not be surprised if oop was more common than oops, at least until oops became part of a set exclamation.