Here is a list of all functions with links to the files they belong to:

on_child_added() : container.c
on_child_removed() : container.c
on_destroy() : widget.c
on_destroy_wrapper() : widget.c
on_match_func() : gtkentry.c
on_match_select() : gtkentry.c
on_screen_member() : lua/lib.c
on_view_changed() : lua/view.c
onsave_action_toggle_callback() : disk.c
opacity_masks() : colorbalancergb.c
open_booking_callback() : help.c
open_chat_callback() : help.c
open_cht() : chart/main.c
open_doc_callback() : help.c
open_donate_callback() : help.c
open_forum_callback() : help.c
open_image() : chart/main.c
open_it8() : chart/main.c
open_reference_image() : chart/main.c
open_search_callback() : help.c
open_source_image() : chart/main.c
operation_tags() : rawprepare.c, watermark.c, scalepixels.c, rotatepixels.c, liquify.c, flip.c, lens.cc, crop.c, clipping.c, borders.c, ashift.c
operation_tags_filter() : ashift.c, clipping.c, crop.c
optimize_changed_callback() : channelmixerrgb.c
optimize_database_callback() : run.c
OPTIONAL() : lib_api.h, iop_api.h, view_api.h, lib_api.h, view_api.h, imageio_storage_api.h, iop_api.h, imageio_storage_api.h, iop_api.h, imageio_storage_api.h, imageio_format_api.h, iop_api.h
or_operator() : collection.c
orientation_member() : box.c, separator.c
orientation_toggle_callback() : imageio/format/pdf.c
origin_color_draw() : channelmixerrgb.c
osm_gps_map_zoom_fit_bbox() : map.c
output_callback() : channelmixer.c
output_colorspace() : colorin.c, colorout.c, demosaic.c
output_format() : rawprepare.c
output_power_callback() : filmic.c