I don't think that Felicity is being portrayed as self-centered. I think that her complaints regarding Oliver's secrecy arise from the fact that two people who are supposed to be a couple can't work as a team if they constantly keep important information from each other. It hurts the teamwork, the morale, and the trust needed for a serious relationship.
And I think Felicity got upset because this wasn't an isolated incident; it's a recurring problem that Oliver has. I'm not saying Oliver didn't have good reasons to feel overwhelmed by the news that he has a son. I'm just saying that since this is such a huge change in his life, Felicity deserves to know about it because she has already entered into what is supposed to be a serious, long-lasting romantic relationship. It's hard for two people in such a relationship to function well if one of them is unaware of developments as crucial as this one (the existence of a child from a former relationship).
And Oliver's problem with telling the truth is further highlighted in this episode by the fact that he admitted that he didn't even know when he would tell Felicity the news. So for all we know, he could have put off telling her the truth indefinitely until she found out for herself through other means. Things like that tend to hurt teamwork and trust.