Yes, with fly region set ...
App instances, when no region is defined, are created in the region closest to wherever flyctl launch commands are run from.
There’s also the PRIMARY_REGION directive in app config that determines, well, the primary region, but it only works for Fly-automated databases (Postgres).
For Machine apps, set region with the fly m run . --dockerfile </path/to/dockerfile> --region <aws> ... command, instead (ref).
You can assign regional IPs to your Fly app which ensures that traffic ingress-es into the Fly network in a particular region (instead of the nearest Fly edge to the client of your app): Multi-region application where users / sessions are pinned to one region? - #5 by ignoramous
You can also pin traffic to a particular region (in case of multi region apps), see: Fly-Instance-Id header alternative for websockets - #2 by ignoramous
Re: OVH: I don’t think Fly exposes that kind of granularity. I don’t think they ever will.