Pfthb

This is for a C++ project

I used to be able to collapse all functions in my source code with Ctrl + M + O shortcut in VS 2015, but this doesn't work any longer in VS 2017.

Ctrl + M still works to collapse the current function

Can anyone help restore this useful functionality?

In Visual Studio you can explore shortcuts using Tools | Options | Environment | Keyboard.

To see what CTRL+M, CTRL+O does, click on the "Press shortcut keys" box so the cursor is located within it and type CTRL+M, CTRL+O. The text box below will show what the shortcut is assigned to (if anything).

In the screenshot below you can see that my copy of Visual Studio 2017 has the shortcut assigned to Edit.CollapsetoDefinitions in the context of the Text Editor.

I left the context drop-down at "Global". But a shortcut may have different meanings in different contexts. You may need to change the drop-down to "Text Editor" to see the shortcut binding you are interested in.

To find out what shortcut (if any) is assigned to a command click on the "Show commands containing" box to locate the cursor within it and type the command (or part of it). Then select the command you are interested in from the shortlist offered.

In the screenshot below you can see that in my copy of Visual Studio 2017 there is only one command containing "CollapsetoDefinitions" and that command is assigned CTRL+M, CTRL+O.

As suggested in the comments, your profile may have assigned a different meaning to CTRL+M, CTRL+O and/or may have assigned a different shortcut to Edit.CollapsetoDefinitions. You may get some idea of what caused the change by examining your own settings.

You might be able to change your copy of Visual Studio back to using standard settings by using Tools | Import and Export Settings | Reset all settings. But of course you risk losing other shortcuts and settings created by whatever changed your system.

If you choose to take this tack be sure to say "yes" when asked if you want to save your current settings before resetting. That way you can at least restore them if you decide resetting was a bad idea.

Alternatively, if CollapsetoDefinitions is assigned a different shortcut, you could decide to use that shortcut instead of CTRL+M, CTRL+O.

Or, if you don't care about losing the existing definition (if any) for CTRL+M, CTRL+O then you could use the Keyboard Options page to assign that shortcut back to CollapsetoDefinitions. If you choose this last option, you'll probably want to change the "Global" drop-down to "Text Editor" on the Keyboard Options page to limit the scope of your change.