For Java 7 new features, you may want to look up "Java 7 New Features Cookbook" by Richard Reese & Jennifer Reese.
The upgrade objectives clearly list of what the Java 7 "new" features you must know.
Java 8 OCA beta is going to end soon (last time I checked). Java 8 will simply add more new features (eg lambda and new time API) to the mix.
Having Java 7 OCP, moving to Java 8 would be just again learning these new stuff.
In fact earlier in the year I decided to upgrade to Java 7 from Java 5 because:
Java 7 has comparatively many new topics (when compared to Java 6)
Given the exam topics, I have used some of them (eg
JDBC, design
patterns, i18N) so reviewing shouldn't be that hard right?
Get myself in mood for Java EE developer certs