They're similar; both technologies allow clients to specify the data they need. I would say that GraphQL is more flexible and has a strong type system. For example, the filter semantics are defined within OData, while in GraphQL, you define how your data can be filtered within your schema.

As a personal opinion, I also feel OData exposes an API that is too tightly coupled to the persistence layer. GraphQL objects and properties are all backed by arbitrary "resolver" functions, which means you can stitch together multiple/legacy backends to generate your response.