We start by appreciating that as humans, we cannot understand divinity any more than cockroaches can understand humanity,
So the question is: what was Jesus trying to communicate?
Jesus uses this statement of "being one" in key ways to describe unity and togetherness generally.
Jesus uses the same phrase to refer to his followers being united (John 17:11). He specifically says that he wants his followers to emulating the togetherness of Christ and his Father. If this idea only applied to the Father and the Son being identical, asking his Father that his followers could be one as Christ and the Father are one, would not make any sense unless all Christ's followers are also identical to each other.
More illumination is offered in John 17:21 where Christ describes what he means. The KJV translated this verse as:
John 17:21 That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me.
But, a more literal translation is, translating “being one” as “united” would be:
That all these might be united, even as you, Father with me and I with you; that they may also be with us; that the world order might believe you sent me out for this.
You can see how well the concept of being “united" works here. For more on the actual Greek of John 10:30, click here.