If we consider context of computing, what is the opposite of distributed? Is it single or combined ? What is the opposite of "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)
- Location: Distributed ↔️ Co-located
- Control: Decentralized ↔️ Central
I also wrote a longer explanation
Some distributed computing frameworks  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.