Antonym of "distributed" as in "distributed system"

Probably depends on the precise situation, but for written technical use I would go with "co-located". "Distributed" implies different things in different locations, where "co-located" implies different things in the same location. Both of your suggestions actually suggest only a single thing (in one way or another)

In the context of databases and software applications in general, the antonym of distributed is centralized.

Concentrated is an antonym for 'distributed' from its connotation of 'spread out'.

Or perhaps an appropriate word is "localized."

Some distributed computing frameworks [1] also use the term single node setup. This would match a context where distributed mainly refers to the fact that computation runs across multiple compute nodes. In this context, something like local computation might be appropriate as well.

[1] https://community.hortonworks.com/questions/203614/-what-is-cluster-single-node-cluster-and-node.html

• Location: Distributed ↔️ Co-located
• Control: Decentralized ↔️ Central

I also wrote a longer explanation