Information borrowed from
The O'Neill class battleship is an Asgard spacecraft in the science fiction television show, Stargate SG-1. It is a more advanced ship than the Beliskner-class cruiser and was the first class designed primarily for the fight against the Replicators. The hull is made of an alloy of trinium, carbon and Naqahdah.
It was named after Colonel Jack O'Neill of SG-1, as O'Neill has
become legendary among the Asgard as representing the next stage in human
evolution (due to his brain's capability to receive the library of the Ancients
("The Fifth Race".)
The first O'Neill-class battleship, the O'Neill, was used as
a decoy to protect an Asgard planet from three Replicator-infested ships.
The O'Neill then flew on an automated course out of the planet's
orbit and entered hyperspace to make the Replicators think the Asgard were
trying to hide her from them. The Replicator ships took the bait and
changed their course to follow the O'Neill, disengaging the shields
and transferring all power to the engines. After the Replicator ships had
caught up to the O'Neill, it then self-destructed and the explosion
destroyed the three following ships. The plan was all thanks to the
"primitive" ideas of Major Samantha Carter who thought up the
plan on-board Thor's ship.
The O'Neill class is later mass-produced and serves as the main warship
of the Asgard, replacing the Beliskner-class cruiser. Three of them are
used to scare away Anubis, when he threatens to kill SG-1 as well as the Asgard
scientist Heimdall. Thor came in an O'Neill ship to ask SG-1's help to
defeat the Replicators. When the Replicators try to attack the new Asgard
homeworld Orilla, the Asgard dispatch six O'Neill-class ship to destroy the
arriving Replicator ship. The six ships are able to destroy the Replicator ship
when it exits hyperspace but they can't stop the Replicator blocks from raining
down onto the planet.
In the Stargate SG-1 Season 9 finale, two Earth battlecruisers, an O'Neill class ship, and numerous Ha'tak arrive at the Ori's second supergate. When Lt. Colonel Carter is unable to dial out from the gate, the Ori send an attack through it from their home galaxy, resulting in the Battle of P3Y-229. The defending vessels were unable to cause any damage to the Ori ships. Only the two Earth vessels could withstand some direct hits from the Ori main weapon. The Ha'tak shields were unable to withstand the fire and many were lost. It is unknown how well the O'Neill class battleship fared.