change opera file posistion string, save following as a .reg file and execute it. then you can see it in your default program settings in windows 7. reference RegisterFirefoxPortable.exe and https://msdn.microsoft.com/en-us/library/windows/desktop/cc144154(v=vs.85).aspx

Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaHTML]
@="Opera HTML"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaHTML\DefaultIcon]
@=""D:\360\Opera\launcher.exe",0"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaHTML\shell]

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaHTML\shell\open]

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaHTML\shell\open\command]
@=""D:\360\Opera\launcher.exe" -url "%1""

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaURL]
@="Opera URL"
"URL Protocol"=""

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaURL\DefaultIcon]
@=""D:\360\Opera\launcher.exe",0"

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaURL\shell]

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaURL\shell\open]

[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\OperaURL\shell\open\command]
@=""D:\360\Opera\launcher.exe" -url "%1""

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable]
"LocalizedString"="Opera Portable Edition"
@="Opera Portable"

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\Capabilities]
"ApplicationName"="Opera Portable Edition"
"ApplicationDescription"="Opera web browser bundled with a PortableApps.com Launcher as a portable app, so you can take your bookmarks, extensions and saved passwords with you."
"ApplicationIcon"=""D:\360\Opera\launcher.exe",0"
"Hidden"=dword:00000000

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\Capabilities\FileAssociations]
".htm"="OperaHTML"
".html"="OperaHTML"
".shtml"="OperaHTML"
".xht"="OperaHTML"
".xhtml"="OperaHTML"

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\Capabilities\StartMenu]
"StartMenuInternet"="OperaPortable"

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\Capabilities\URLAssociations]
"http"="OperaURL"
"https"="OperaURL"
"ftp"="OperaURL"

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\DefaultIcon]
@=""D:\360\Opera\launcher.exe",0"

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\shell]

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\shell\open]

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\shell\open\command]
@=""D:\360\Opera\launcher.exe""

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\shell\properties]

[HKEY_LOCAL_MACHINE\SOFTWARE\Clients\StartMenuInternet\OperaPortable\shell\properties\command]
@=""D:\360\Opera\launcher.exe" -preferences"

[HKEY_LOCAL_MACHINE\SOFTWARE\RegisteredApplications]
"Opera Portable"="Software\Clients\StartMenuInternet\OperaPortable\Capabilities"

It is cited by Opera that portable version does not support this. But I cannot help requesting this feature. Yes, Chromium may not support this. But it is my wish. There is even a program to do this for portable firefox.