Proof of copyrightability is easy to come by. Just see Johnson Controls v. Phoenix Control Systems
where it says:
(12) Source and object code, the literal components of a program, are consistently held protected by a copyright on the program. See, e.g. CMS Software Design Sys., Inc. v. Info Designs, Inc., 785 F.2d 1246, 1249 (5th Cir.1986) (source code); Apple, 725 F.2d 521 (object code). Whether the non-literal components of a program, including the structure, sequence and organization and user interface, are protected depends on whether, on the particular facts of each case, the component in question qualifies as an expression of an idea, or an idea itself.
(13) Here, the district court found that the structure, sequence and organization of the JC-5000S was expression, and thus subject to protection. The JC-5000S is a very sophisticated program, and each individual application is customized to the needs of the purchaser. This practice of adaptation is one indication that there may be room for individualized expression in the accomplishment of common functions. The district court's finding of expression is also supported by the special master's report. Regarding one particular point type, the master indicated that, although it is common for process control software packages to include provisions for collecting historical data, and using various integration and averaging schemes to do so, it is unusual to implement this function as a point type, as Johnson did. This finding also supports the view that some discretion and opportunity for creativity exist in the structure, and that the structure of the JC-5000S is expression, rather than an idea in itself. This issue will no doubt be revisited at trial, but at this stage of the proceedings we cannot say that the district court clearly erred.
And there you have it. The API is part of the structure, sequence and organization and user interface
and therefore is potentially copyrightable. However actually achieving copyright protection requires a higher bar than for just copying literal text.
In Oracle's case the bar is going to be even higher. In their case even if the court rules that it was copyrightable, Google can argue that Oracle has years of telling people that they have no copyright interest on code that has copied elements of those APIs in their own code (by using them), and therefore by the doctrine of unclean hands can't now turn around and sue someone over that copyright.
As always I am not a lawyer and this is not legal advice. It is merely my lay opinion that there are court rulings that support the claim that APIs are potentially copyrightable, but the bar to winning copyright is very high, and even if Oracle is ruled to have copyright, they still are likely to lose their case