Just refreshed org from the package manager.

  org                20190415      installed             Outline-based notes management and organizer
  org-plus-contrib   20190415      installed             Outline-based notes management and organizer

org-drill-entry: Symbol’s function definition is void: org-outline-overlay-data

I was able to fix the problem by grepping for references in *.elc files and byte-compiling the files.

sloth% ls -lrt | tail -8
-rw-r--r--  1 dap  staff    12041 Apr 15 21:36 ox-rss.elc
-rw-r--r--  1 dap  staff    13775 Apr 15 21:36 ox-s5.elc
-rw-r--r--  1 dap  staff    29636 Apr 15 21:36 ox-taskjuggler.elc
-rw-r--r--  1 dap  staff    58283 Apr 15 21:36 ox-texinfo.elc
-rw-r--r--  1 dap  staff   242279 Apr 15 21:36 ox.elc
-rw-r--r--  1 dap  staff   100111 Apr 18 06:10 org-drill.elc
-rw-r--r--  1 dap  staff   102041 Apr 18 06:11 ob-core.elc
-rw-r--r--  1 dap  staff   796772 Apr 18 06:11 org.elc
sloth% pwd
/Users/dap/.emacs.d/elpa/org-plus-contrib-20190415
sloth%