Simply put, if you are a fan of Fate Grand Order this OVA is pure entertainment. Getting to see a plethora of beloved servants animated, being goofy, and messing around with each other makes for a hysterical and amazing show. Ritsuka (orange-haired MC) is surprisingly funny as well, as they have taken the occasionally sarcastic/troll aspect of the somewhat self-insert MC from the game, dialed it up to a 10, and combined it with the FGO player experience to make for some hilarious scenes. Many of the jokes in general end up breaking the 4th wall and are extremely self-aware, making references to the game’s meta, farming, unit rankings, even the garbage summoning system itself. With the inclusion of classic OSTS and UI from the game, the show carries a strong FGO identity, enhancing the viewing experience and capitalizing on the viewer's connection to the game. It feels like the staff behind this OVA are very passionate about the game themselves, and it’s clear that they put a lot of love into this.
It’s painfully short
For a show that relies heavily on fanservice and series attachment, the runtime ends up being a weakness, spanning only two 30 minute episodes (or four 15 minute eps). Due to the immense size of the cast, it’s inevitable that many characters merely end up being extras or not appearing at all ~~(WHERE'S USHI AND SCATHACH)~~. Some characters like Ereshkigal and Jeanne Alter have a solid amount of screentime but you are only getting crumbs for others like Carmilla or Fuuma. With the amount of content in FGO it would be easy to create a 12-24 episode comedy series, giving some of the less relevant characters more spotlight and allowing some of the skits more room to breathe. Doesn’t necessarily mean the OVA is flawed, just that it would be more effective as fan-service if it had more time. Hopefully they continue to make more episodes in the future and bring in some of the newer servants as well.
Can I watch it without watching/playing FGO?
Unfortunately this anime is not very accessible to people who don't know much about FGO. With the sheer number of servants, references, and inside jokes scattered throughout the episodes, a lot of gags might go over your head if you aren't familiar with the game. The entertainment value scales with your game knowledge and attachment to the franchise, so while you can probably go in blind and enjoy it, it most likely won't hit as hard because you'll likely feel that you're missing the full package. The OVA's and movies can help bridge the gap somewhat but it seems clear that they were targeting this OVA towards more experienced players.
TLDR:
The more knowledgeable you are about the Fate Grand Order franchise and the more you like it, the more you will get out of this OVA. For a show that is meant to be pure fanservice for fans of the game it’s amazing, and feels like a big thank you letter to the community for helping make this possible. It is definitely a MUST-WATCH if you are a fan of FGO. For those not as familiar with FGO, in case you haven't watched it I recommend Carnival Phantasm. It is much more accessible to the average Fate fan, and even if you aren't acquainted with the Tsukihime characters in there it's still a good time.