The Ori Mothership is a type of starship that features in Stargate SG-1.
The Ori motherships first appeared under construction in the episode "Crusade" and invaded the Milky Way galaxy in the following episode "Camelot". Four of these ships were responsible for the destruction of a combined Asgard, Human, Jaffa, Lucian Alliance and Tok'ra fleet that attempted to prevent the Ori invasion at the Battle of P3Y-229. Aboard one of the battlecruisers was Vala Mal Doran, who had been stranded in the Ori galaxy.
After defeating the allied fleet at the Supergate, the Ori motherships made their way to the Free Jaffa Nation world of Chulak where they quickly gained control. After orbiting Chulak one ship that carried the Orici landed while the other remained in orbit and launched their fighters. The Jaffa were powerless against this invasion force with none of their weapons being effective. Even ramming a Ha'tak class warship directly into an Ori mothership was not successful. The Ori continued their efforts in converting the galaxy and it was stated that since the crusade began, six more worlds had fallen.
In the episode "The Pegasus Project", SG-1 managed to access the Supergate from the Pegasus Galaxy, at least temporarily preventing the Ori from sending additional forces. In the process, one of the four Ori motherships already in the Milky Way was destroyed by the unstable vortex (the kawoosh) of the activating Supergate. This left only three battlecruisers within the Milky Way galaxy,
A few weeks later, an Ori mothership was encountered landed on a recently converted planet inhabited by thousands of people. This particulair ship also carried Adria who was spreading Origin to the locals when an energy wave from the Dakara Superweapon destroyed all organic life on the planet except for the Orici herself. The ship was nearly captured by SG-1 and Free Jaffa, who were at odds for control of the vessel until Adria arrived to take control once again. This lone ship devastated all Jaffa opposition in its path as it laid waste to the Free Jaffa Nation homeworld of Dakara. 
Several months later, Daniel Jackson/Merlin was transformed into a Prior so he could steal an Ori mothership. It was his intention to fly it to the Ori home galaxy, along with the Sangraal to use against the Ori. Although both the ship and the active weapon were successfully sent to the Ori home galaxy, the plan involved shutting down the Supergate, allowing six more ships to enter the Milky Way through an incoming wormhole, bringing the total number of known active battlecruisers in the galaxy to eight. 
In the episode "Unending", several Ori ships had discovered the new Asgard homeworld, Orilla. There one was destroyed by the Earth ship Odyssey through use of tacked-on Asgard energy weapons and two were probably vaporized by Orilla's self-destruction. However, a few were able to track the Odyssey through its new hyperdrive and after the Earth ship destroyed another mothership the remaining ships were about to destroy it when Carter threw a time dilation field around it thus trapping the ship.
Toward the end of the episode, Carter manages to redirect the energy of the beam into the Asgard power core reversing time back to the moment she activated the dilation field. Teal'c stays old holding a program control crystal meant to disconnect the Asgard computer from the hyperdrive. After the time reversal is complete Tealc installs the crystal and prevents Carter from activating the dilation field. Odyssey, now untrackable by the Ori, jumps to hyperspace out of the way of the incoming Ori energy beam.
This brings the total number of Ori motherships destroyed to four. The Apollo and the Daedalus were eventually retrofitted with the Asgard technology, as was the Phoenix in the alternate timeline established in "The Last Man".
This places the Tauri ships on par, if not superior, to the Ori motherships. The Odyssey shields were capable of taking a hit from the Ori mothership's main cannon, losing only 23% of their shield strength. The Odyssey, in "Unending" suffered total shield failure after the fourth hit, outmanuevered and had four times the firing rate of each of the Ori motherships it went up against.
Little is known about the Ori ship's technology or capabilities. What is known is that it has extremely powerful shields, protecting it from all but the most powerful attack. The vessels are powered by a central power core, and are mostly crewed by ordinary humans, with Priors controlling the main systems. The bridge of the Battlecruiser has a control chair that resembles the chairs in Atlantis and Ancient outposts. This has been theorized to mean that only a Prior can operate and directly control the ship. However, Adria's Orici abilities allow her to control an Ori ship without use of the chair. 
Aboard, there are prayer rooms where followers worship the Ori, armories, and quarters for the crew. The bridge simply consists of a computer console on one side, the mentioned control chair, and a view screen. The inner halls are structurally simple and tinted blue. The doors are controlled by a nearby sensor which is waved at to open the door, similar to the doors on Atlantis. The central power core of the ship consists a large room with a device, bearing a striking resemblance to the abandoned Ancient project Arcturus, that glows with an internal white light and shaped liked the symbol of Origin. It is still unknown what the energy source is.
The interior section of the ship is completely hollow where the hull curves around what appears to be a device based on the Stargate's design that glows white when powered. It was these devices that were seen to initialize when the Priors activated the ships in "Crusade". However, "Counterstrike" indicated the ship was powered by a device contained in a sealed chamber, suggesting the ring-like device was not powering the vessel.
The Battlecruiser appears to have one main weapon, housed at the base of the bow, that can destroy a Goa'uld Ha'tak mothership with a single blast or a Daedalus class battlecruiser in at least two simultaneous direct hits from two different Motherships. In addition to the massive main cannon, the ship is also equipped with rapid-fire energy pulse cannons spread about the hull. A Ha'tak appeared capable of surviving a few hits from these weapons, but not without taking heavy damage.  The battlecruiser also carried a complement of smaller ships, fighters, similar in size to an Al'kesh which battled Death Gliders and Al'kesh above Chulak. This means that like most of the races in the Stargate universe, the Ori ships have a carrier style function. Notable exceptions are the Replicators, the Asgard, and the Ancients, who do not appear to use fighters (Puddle Jumpers are too few in number to have originated as a fighter wing; rather, they are armed shuttles). The Ori mothership has a functioning hyperdrive to travel between stars.  However, the only known way these vessels can move between the Ori galaxy and the Milky Way is by use of an Ori Supergate, implying that the Ori galaxy is too far away from the Milky Way for practical hyperspace travel.  This is further demonstrated when Ori battlecruisers follow the Tau'ri ship Odyssey to the Asgard homeworld of Orilla which suggests the use of intergalactic hyperdrives.  The ship also has transmission systems capable of translating and transmitting, at the least, text messages to other spacecraft. 
For its size, the Ori mothership is superior to any ship known to the SGC. The Wraith Hive-Ship, Ancient City Ship and Aurora class battleship are probably the only ships comparable to the Ori mothership in size.
- Stargate SG-1: