[LG G5 Questions & Answers] tremalnaik:
have you already tried to download the bootloader flashable zip for H850 Oreo? It contains many necessary partitions all as .img files packed in one zip. You may try to flash them using fastboot, but I really don't have any idea of the specific command to tell fastboot where should they be flashed (i.e. I know "fastboot flash recovery recovery.img", but I don't know "fastboot flash ??? bootloader.img").

EDIT: you can see here (in the updater-script) where each partition has to be flashed (where the package_extract_file command is executed) .

ui_print("Flashing Bootloader H850 30a Oreo");
package_extract_dir("bootloader", "/tmp");
set_perm_recursive(0, 0, 0777, 0777, "/tmp");
ui_print("Verifying ZIP contents...");
ui_print(" ");
apply_patch_check("/tmp/aboot.img", "593cdaf9b8d7f0b429c8735384171b75273e9aba") || abort("\"/tmp/aboot.img\" has failed verification.");
apply_patch_check("/tmp/apdp.img", "3c0ebb5d9232a35d863dc3c6c57e743403a87027") || abort("\"/tmp/apdp.img\" has failed verification.");
apply_patch_check("/tmp/cmnlib.img", "fe997d579eb9aea77e9284ed2f616c5003665950") || abort("\"/tmp/cmnlib.img\" has failed verification.");
apply_patch_check("/tmp/cmnlib64.img", "9ce464a1778b9d67a877c5e38179c2aa8c83c7c1") || abort("\"/tmp/cmnlib64.img\" has failed verification.");
apply_patch_check("/tmp/devcfg.img", "f2861ead5134b660197218881cafbea2cdfb154b") || abort("\"/tmp/devcfg.img\" has failed verification.");
apply_patch_check("/tmp/factory.img", "b6be0111a1fdfee15608260f93c7931402d92cb7") || abort("\"/tmp/factory.img\" has failed verification.");
apply_patch_check("/tmp/hyp.img", "a0e072a0af7a893ec146ff40be4462b9d8a287fa") || abort("\"/tmp/hyp.img\" has failed verification.");
apply_patch_check("/tmp/keymaster.img", "9b985259de91dce6b7dbb29153fd8be12bcd5ee7") || abort("\"/tmp/keymaster.img\" has failed verification.");
apply_patch_check("/tmp/laf.img", "a62db890a1202060d39e26cf1561c2f34fe74a66") || abort("\"/tmp/laf.img\" has failed verification.");
apply_patch_check("/tmp/msadp.img", "a965d9247f269ce4cf0f6ea219f822a3e83ef36d") || abort("\"/tmp/msadp.img\" has failed verification.");
apply_patch_check("/tmp/pmic.img", "c367fe01778c981889a5a9f53eebc6d1c1e1c0e3") || abort("\"/tmp/pmic.img\" has failed verification.");
apply_patch_check("/tmp/raw_resources.img", "39d23e6fe83c4fba7914885e60baec99d8bda66d") || abort("\"/tmp/raw_resources.img\" has failed verification.");
apply_patch_check("/tmp/rpm.img", "d2fdadd1986f247904f5a9e9cb3b67491678274f") || abort("\"/tmp/rpm.img\" has failed verification.");
apply_patch_check("/tmp/sec.img", "5567df929c7d846e84bf409c96e90be9a82e54e1") || abort("\"/tmp/sec.img\" has failed verification.");
apply_patch_check("/tmp/tz.img", "3d8b5e060de01c919a6c8c771eefa06f9292b837") || abort("\"/tmp/tz.img\" has failed verification.");
apply_patch_check("/tmp/xbl.img", "d500c72ee5210da4ced66702dabf3a4bbbeb345c") || abort("\"/tmp/xbl.img\" has failed verification.");
ui_print("Flashing. this might take a bit...");
ui_print(" ");
package_extract_file("bootloader/aboot.img", "/dev/block/bootdevice/by-name/aboot");
package_extract_file("bootloader/aboot.img", "/dev/block/bootdevice/by-name/abootbak");
package_extract_file("bootloader/apdp.img", "/dev/block/bootdevice/by-name/apdp");
package_extract_file("bootloader/cmnlib.img", "/dev/block/bootdevice/by-name/cmnlib");
package_extract_file("bootloader/cmnlib.img", "/dev/block/bootdevice/by-name/cmnlibbak");
package_extract_file("bootloader/cmnlib64.img", "/dev/block/bootdevice/by-name/cmnlib64");
package_extract_file("bootloader/cmnlib64.img", "/dev/block/bootdevice/by-name/cmnlib64bak");
package_extract_file("bootloader/devcfg.img", "/dev/block/bootdevice/by-name/devcfg");
package_extract_file("bootloader/devcfg.img", "/dev/block/bootdevice/by-name/devcfgbak");
package_extract_file("bootloader/factory.img", "/dev/block/bootdevice/by-name/factory");
package_extract_file("bootloader/hyp.img", "/dev/block/bootdevice/by-name/hyp");
package_extract_file("bootloader/hyp.img", "/dev/block/bootdevice/by-name/hypbak");
package_extract_file("bootloader/keymaster.img", "/dev/block/bootdevice/by-name/keymaster");
package_extract_file("bootloader/keymaster.img", "/dev/block/bootdevice/by-name/keymasterbak");
package_extract_file("bootloader/laf.img", "/dev/block/bootdevice/by-name/laf");
package_extract_file("bootloader/msadp.img", "/dev/block/bootdevice/by-name/msadp");
package_extract_file("bootloader/pmic.img", "/dev/block/bootdevice/by-name/pmic");
package_extract_file("bootloader/pmic.img", "/dev/block/bootdevice/by-name/pmicbak");
package_extract_file("bootloader/raw_resources.img", "/dev/block/bootdevice/by-name/raw_resources");
package_extract_file("bootloader/raw_resources.img", "/dev/block/bootdevice/by-name/raw_resourcesbak");
package_extract_file("bootloader/rpm.img", "/dev/block/bootdevice/by-name/rpm");
package_extract_file("bootloader/rpm.img", "/dev/block/bootdevice/by-name/rpmbak");
package_extract_file("bootloader/sec.img", "/dev/block/bootdevice/by-name/sec");
package_extract_file("bootloader/tz.img", "/dev/block/bootdevice/by-name/tz");
package_extract_file("bootloader/tz.img", "/dev/block/bootdevice/by-name/tzbak");
package_extract_file("bootloader/xbl.img", "/dev/block/bootdevice/by-name/xbl");
package_extract_file("bootloader/xbl.img", "/dev/block/bootdevice/by-name/xblbak");
package_extract_file("bootloader/xbl.img", "/dev/block/bootdevice/by-name/xbl2");
package_extract_file("bootloader/xbl.img", "/dev/block/bootdevice/by-name/xbl2bak");
ui_print(" ");
ui_print(" ");
ui_print(" ");
ui_print(" Finished... ");
ui_print(" ");
ui_print("Zip by DavidFlint");
ui_print("Template by autoprime - thx");
ui_print(" ");

[/SPOILER]