A month ago Opera for Linux 24 was released. Unfortunately it was released only as a 64 bit deb package. Thats not a problem because an ebuild was already written (details: bug#514696).

On Sabayon:

Add PORTDIR_OVERLAY="${PORTDIR_OVERLAY} /usr/local/portage/" to /etc/make.conf

# make an ebuild
mkdir -p /usr/local/portage/www-client/opera-developer
cd /usr/local/portage/www-client/opera-developer
wget -O opera-developer-24.0.1558.21.ebuild https://bugs.gentoo.org/attachment.cgi?id=381750
ebuild opera-developer-24.0.1558.21.ebuild manifest

# install:
emerge -av opera-developer


# correct symbolic link to libudev
rm  /usr/lib64/opera-developer/lib/libudev.so.0
ln -s /usr/lib64/libudev.so.1 /usr/lib64/opera-developer/lib/libudev.so.0

Based on:
HOWTO: Example of Installing 3rd Party Ebuilds

HOWTO: Safely mix Entropy and Portage

if You get any errors when generating the manifet file try emerge --sync and make sure are the dependencies are installed.

Installing opera on other systems: Manual Opera installation

Opera 25 is out!

Information about new version:

If You have the previous version installed just make a new ebuild and install:

cd /usr/local/portage/www-client/opera-developer/
cp opera-developer-24.0.1558.21.ebuild opera-developer-25.0.1597.0.ebuild 
ebuild opera-developer-25.0.1597.0.ebuild manifest

#emerge
emerge -av opera-developer

#correct the symbolic links
rm  /usr/lib64/opera-developer/lib/libudev.so.0
ln -s /usr/lib64/libudev.so.1 /usr/lib64/opera-developer/lib/libudev.so.0

rm /usr/bin/opera-developer
ln -s /usr/lib64/opera-developer/opera-developer /usr/bin/opera-developer