My answer is based on the link provided in the question above
What is OS optimized defaults?
The computers come with a default settings for the UEFI (or BIOS). In this case, it looks like the defaults are different for 64bit Windows 8 and above (the OS optimized default) as compared with older versions of Windows.
What does it mean?
Some times one needs to update the UEFI (also called update the BIOS). In some of these cases it is recommended to revert to the default settings after the update and then manually customize any settings you had changed before.
What happens when you press the button to reset UEFI to its default setting depends on whether the the OS optimized default is enabled or not. If enabled the UEFI will default to one set of settings. If disabled, UEFI will revert to a different default. In either case, you should be able to make any changes from the default manually and save those changes in the UEFI.
How different are these two defaults?
There are 5 differences, two related to security and three related to startup as seen in the link above and copied as an image below:
Lets go through this list one by one.
- UEFI BIOS Update Option: Secure RollBack Prevention. I have no idea what it does. I guess it should not have any effects on Ubuntu either way.
- Secure Boot. This is the only thing that may matter for Ubuntu. If your computer has any hardware that needs proprietary drivers, those drivers cannot be installed if secure boot is enabled. (I faced this problem a couple of years ago, things may have improved since then.). So if the default setting enables secure boot, under this circumstances, you will have to remember to disable secure boot anytime you press the button to reset UEFI to its default setting, if OS optimized defaults is enabled.
- UEFI/Legacy Boot. The
Legacy boot is when the new UEFI acts like the old BIOS. In this boot mode, the new features of UEFI are not present. Any OS installed in one boot mode will not boot in the other boot mode. Since you are dual-booting, both OSes must use UEFI mode or both must use Legacy mode. The option to change from UEFI to Legacy is not presented if OS optimized default is enabled. The only choice is UEFI mode. This should not affect installing Ubuntu, as long as you have installed Windows in UEFI mode, and remember to install Ubuntu in UEFI mode as well.
- UEFI/Legacy Boot Priority. This should not matter as long as you install both Windows and Ubuntu in UEFI mode.
- CSM Support. I am not sure what this one does. My guess is, it has to do something with the Legacy mode. Assuming you are using UEFI boot mode for both Windows and Ubuntu, this should not matter.
If you have already installed Windows in Legacy mode, then you want to install Ubuntu in Legacy mode as well. In this case, you should disable OS optimized defaults. Otherwise you may not get the option to boot to Legacy mode, after you reset the UEFI to default for any reason.
If you installed Windows in UEFI mode, then it does not matter, except as mentioned in bullet point 2 above.
Hope this helps