How do i access oculus username?

Getting the username of a player is part of the Oculus Platform SDK. The documentation on how to use it in Unity can be found here. The most important prerequisite is that you'll need an AppID by creating an application in the Developer Dashboard.

To start using the Platform SDK, you'll first have to do an entitlement check as outlined here.

Using either Platform.Core.Initialize(appID) or Platform.Core.AsyncInitialize(appID)

Then, you'll be able to use Platform.Users.GetLoggedInUser() to retrieve a username.

using Oculus.Platform;
using Oculus.Platform.Models;
using UnityEngine;

public class PlatformSDKTest : MonoBehaviour {

  private void GetUserName() {
     Oculus.Platform.Users.GetLoggedInUser().OnComplete(GetLoggedInUserCallback);
  }

  private void GetLoggedInUserCallback(Message msg) {
    if (!msg.IsError) {
      User user = msg.GetUser();
      string userName = user.OculusID;
      string displayName = user.DisplayName;
    }
  }
}

Is there any specific error you're getting? I'm not sure which step you're stuck on.

Make sure you've added the AppID in the OculusPlatformSettings file in Unity. To be safe, I'd also upload an initial build to a release channel in the Oculus Developer Dashboard.

This blog post might also be helpful to you.