I have the same error on Ubuntu LTS 14.04:

~$ sudo pecl install oci8
No releases available for package "pecl.php.net/oci8"
install failed

Finally the reason is the Proxy problem: my computer is sitting after the company proxy server. Run the following command to add the proxy

~$ sudo pear config-set http_proxy http://proxy.mycompany.com:8080/
config-set succeeded

Then run the command again, it works as bellow

~$ sudo pecl install oci8
downloading oci8-2.0.8.tgz ...
Starting to download oci8-2.0.8.tgz (190,854 bytes)
....done: 190,854 bytes
11 source files, building
running: phpize
Configuring for:
PHP Api Version:         20121113
Zend Module Api No:      20121212
Zend Extension Api No:   220121212

Here we type in the Oracle Instant client folder as bellow:

Please provide the path to the ORACLE_HOME directory. Use 'instantclient,/path/to/instant/client/lib' if you're compiling with Oracle Instant Client [autodetect] : instantclient,/opt/oracle/instantclient_12_1

Then the command line continue ouputs:

building in /tmp/pear/temp/pear-build-rootJafknN/oci8-2.0.8
running: /tmp/pear/temp/oci8/configure --with-oci8=instantclient,/opt/oracle/instantclient_12_1
checking for grep that handles long lines and -e... /bin/grep
checking for egrep... /bin/grep -E
...
...
Build process completed successfully
Installing '/usr/lib/php5/20121212/oci8.so'
install ok: channel://pecl.php.net/oci8-2.0.8
configuration option "php_ini" is not set to php.ini location
You should add "extension=oci8.so" to php.ini

Good luck.