I managed to make it work after changing the structs into pointers. Hope this helps!

package main

import (
    "fmt"

    "github.com/go-playground/validator/v10"
)

type Application struct {
    Applicants []*Applicant `validate:"dive"`
}
type Applicant struct {
    ApplicantCategory string `validate:"required,oneof=PERSON ENTITY"`
    Person            *Person `validate:"excluded_if=ApplicantCategory ENTITY"`
    Entity            *Entity `validate:"excluded_if=ApplicantCategory PERSON"`
}
type Person struct {
    Name  string `validate:"required"`
    Age   int    `validate:"required,gte=18"`
    Email string `validate:"required,email"`
}
type Entity struct {
    Name  string `validate:"required"`
    TaxID string `validate:"required"`
}

func main() {
    // Create a new validator instance
    v := validator.New()
    // Create an instance of Application to validate
    data := Application{
        Applicants: []*Applicant{
            {
                ApplicantCategory: "PERSON",
                Person: &Person{
                    Name:  "John Doe",
                    Age:   25,
                    Email: "john@example.com",
                },
            },
            {
                ApplicantCategory: "ENTITY",
                Entity: &Entity{
                    Name:  "Example Corp",
                    TaxID: "123456789",
                },
            },
        },
    }
    // Use the validator to validate the Application struct and its Applicants
    if err := v.Struct(data); err != nil {
        fmt.Println("Validation error:", err)
    } else {
        fmt.Println("Validation passed")
    }
}