I think Oz absolutely deserved to know that Willow was dating someone new, but Willow didn't need to blurt it out as soon as he arrived. As others said, he had gone-and she thought it was for good. She wasn't prepared to tell him anything, or to even consider having him in her life again. So it's perfectly understandable that she would need some time to tell him; heck, I think she was so busy just processing the fact that he was back, and everything he was telling her about learning to control the wolf, that she gets a pass here.
Also, it must have been very confusing for her considering she'd just started dating Tara, but had loved Oz for years. And I'm sure she wasn't in a hurry to alienate him--something she knew would happen once she told him she'd moved on. And he, of course, shouldn't have expected her to have waited for him.