Usually, by default on Linux servers SSH service along with other packages are installed and enabled, Opera VPS also provides Linux VPS with SSH service by default and you don't need to install it, but if you need to install Enter the following command in Centos/Fedora servers:
yum -y install openssh-server openssh-clients
Run the following command on Ubuntu/Debian/Linux Mint servers:
sudo apt-get install openssh-server openssh-client
If you need more guidance about Enabling SSH in Ubuntu, read the How to Enable SSH on Ubuntu article.