This is what grep tells me about non-changelog mentions of liboil in gst*:

gstreamer/docs/faq/dependencies.xml - mentions liboil as one of the mandatory dependencies

gst-plugins-base/tests/check/elements/ffmpegcolorspace.c - sets OIL_CPU_FLAGS (but doesn't use liboil)

gst-plugins-base/gst/audioconvet/gstchannelmix.c - mentions that liboil might be used to optimize inner loops

gst-plugins-base/gst/playback/gstplaysink.c - mentions liboil in some kind of error message.

win32 stuff in gst-plugins-base contains liboil .lib file references

gst-plugins-base/tests/check/elements/videotestsrc.c - sets OIL_CPU_FLAGS (but doesn't use liboil)

gst-plugins-good/gst/level/gstlevel.c - a commented-out calls to liboil

win32 stuff in gst-plugins-good contains liboil .lib file references

gst-plugins-good/tests/check/elements/videocrop.c - sets OIL_CPU_FLAGS (but doesn't use liboil)

gst-plugins-ugly/ext/a52dec/gsta52dec.c - logs "liboil" as a source of cpu flags (but uses orc to get them)

gst-plugins-ugly/gst/dvdsub/gstdvdsubdec.c - TODO: liboil-ise code, esp. use _splat() family of functions

gst-plugins-ugly/REQUIREMENTS - claims that it requires liboil >= 0.2.0

gst-plugins-bad/ext/cog/cogframe.c - liboil calls in commented-out chunks of code

gst-plugins-bad/gst-plugins-bad.spec.in - claims that it requires liboil-devel

gst-plugins-bad/ext/dts/gstdtsdec.c - logs "liboil" as a source of cpu flags (but uses orc to get them)

gst-plugins-bad/ext/cog/gstlogoinsert.c  - uses oil_divide_255(x), which is just a define for ((((x)+128) + (((x)+128)>>8))>>8)

gst-plugins-bad/gst/games/gstpuzzle.c - if compiled with liboil support, calls oil_init
gst-plugins-bad/gst/games/gstvideoimage.c - if compiled with liboil support, includes liboil/liboil.h and calls oil_splat_u8(), otherwise uses a C backup implementation
gst-plugins-bad/gst/games/Makefile.am - adds $(LIBOIL_LIBS) to LDFLAGS for libgstpuzzle
gst-plugins-bad/ext/directfb/Makefile.am - adds $(LIBOIL_LIBS) to LIBADD for libgstfbvideosink

gst-plugins-bad/ext/wayland/Makefile.am - adds $(LIBOIL_LIBS) to LIBADD for libgstwaylandsink

gst-ffmpeg/gst-ffmpeg.spec.in - claims that it requires liboil-devel >= 0.3.6, and that is the only place where liboil is mentioned in gst-ffmpeg


That is, only bad/gst/games actually uses liboil.