[Nokia 3 Guides, News, & Discussion] bigrammy:
IDK much about OST as it's not my preferred method of flashing firmware I did have some experience of it when I used my Infocus M808 (FIH's own Brand) but that device had a separate minimal boot.img and system.img who's sole purpose was so OST would work :silly:

I never understood why they use it on MediaTek but hey ho there you go. I simply got nb0 file extracted it and then used SPFlashTool.

Some clues as to what the OST and FIH's binary's do are contained in the .mlf file.

I have highlighted in red the relevant part's

[Package Download Profile]
PROFILE_VER = 1.0
MLF_VER = 1.0
PROJECT = "NE1"
PLATFORM = "M5"
PACKAGE_NUMBER = 18
INITOPTION =0x0
INITDLTYPE = 0x20
CHIP_NAME = MT6737

[Package Info Main]

[Package Info 1]
PACKAGE_NAME = "scatter"
BOOT_NAME = "scatter"
OPTION = 0x1
IMAGE_FILE = "NE1-0-1301-MT6737M_Android_scatter.txt"

[COLOR="Red"][B][Package Info 2]
PACKAGE_NAME = "oem enable security"
BOOT_NAME = "oem disable security"
OPTION = 0x1000000000000000
IMAGE_FILE = ""[/B][/COLOR]

[Package Info 3]
PACKAGE_NAME = "PRELOADER"
BOOT_NAME = "preloader"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-preloader_NE1.bin"

[Package Info 4]
PACKAGE_NAME = "LK"
BOOT_NAME = "lk"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-lk.bin"

[Package Info 5]
PACKAGE_NAME = "tee1"
BOOT_NAME = "tee1"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-trustzone.bin"

[Package Info 6]
PACKAGE_NAME = "tee2"
BOOT_NAME = "tee2"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-trustzone.bin"

[B][COLOR="red"][Package Info 7]
PACKAGE_NAME = "reboot-bootloader"
BOOT_NAME = "reboot-bootloader"
OPTION = 0x1000000000000000
IMAGE_FILE = ""[/COLOR][/B]

[Package Info 8]
PACKAGE_NAME = "BOOT"
BOOT_NAME = "boot"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-000C-boot.img"

[Package Info 9]
PACKAGE_NAME = "RECOVERY"
BOOT_NAME = "recovery"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-000C-recovery.img"

[Package Info 10]
PACKAGE_NAME = "SECRO"
BOOT_NAME = "secro"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-secro.img"

[Package Info 11]
PACKAGE_NAME = "LOGO"
BOOT_NAME = "logo"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-logo.bin"

[Package Info 12]
PACKAGE_NAME = "ANDSYSIMG"
BOOT_NAME = "system"
OPTION = 0x2
IMAGE_FILE = "NE1-0-1301-000C-system.img"

[Package Info 13]
PACKAGE_NAME = "sutinfo"
BOOT_NAME = "sutinfo"
OPTION = 0x2
IMAGE_FILE = "NE1-00WW-001-sutinfo.img"

[Package Info 14]
PACKAGE_NAME = "hidden"
BOOT_NAME = "hidden"
OPTION = 0x2
IMAGE_FILE = "NE1-0-0010-0001-hidden.img.ext4"

[Package Info 15]
PACKAGE_NAME = "cda"
BOOT_NAME = "cda"
OPTION = 0x2
IMAGE_FILE = "NE1-000C-001-cda.img"

[Package Info 16]
PACKAGE_NAME = "sys_info"
BOOT_NAME = "sys_info"
OPTION = 0x2
IMAGE_FILE = "fver"

[Package Info 17]
PACKAGE_NAME = "CACHE"
BOOT_NAME = "cache"
OPTION = 0x4000
IMAGE_FILE = "cache.img"

[Package Info 18]
PACKAGE_NAME = "USERDATA"
BOOT_NAME = "userdata"
OPTION = 0x4000
IMAGE_FILE = "userdata.img"

As for the bootloader I did already explain this once so put very simply If you have unlocked the bootloader before you are still bootloader unlocked even after flashing with OST .nb0 or SPFlashTool as said I am unsure which partition contains the bootloader unlocked and tamper flag bites maybe secro or some other partition that does not get overwrote using the above tools.