["Human nature is selfish"]

Is there such as thing as "human nature," and if so, is it accurate to state that it is selfish?

What do you mean by "nature" and by "selfishness?" By nature, do you mean "metaphysical material identity" or are you referring to a pre-birth or genetic mental endowment? The clearest answer in Objectivism requires this distinction. Otherwise you have to read thru questions about the place of biology and drug use as concrete examples of questions.

['Everything is one' view]

The idea that reality is the one in the many was first proposed by Thales, thought by many to be the first philosopher.  So, if you are new to philosophy, perhaps you are asking a good first question.  When participating in discussions with others who propose this idea of "oneness" you might ask, "what do you mean by "oneness"?"  Is it the material basis of matter you are asking about (and wondering how to think about the fact that direct/unaided human senses alone cannot perceive it) or are you proposing something about unity of form in non-material, derivative real things like human concepts?

 

The answer you would find in Objectivism depends, in part, to the real nature of the question being asked.  Because so few people you meet have any fundamental basis for their questions or conclusions, step one should be to get that person to be more specific about the question.