Can a voice recorded conversation be used in a court, if I don't tell my boss I am recording our conversation?

Yes you do., but check the state laws. In some states only one person has to be aware of the recording, and I know that doesn't make much sense, but it is true in some jurisdictions.

Check out this blog post, http://legallad.quickanddirtytips.com/the-legality...

It does a good job going over the basic of the law.

The law concerning the rectitude of surveillance will control the admissibility of the recording. However, since you use the term 'admissible' (an concept from the law of evidence) I will also say that this recording, if permitted, will qualify as an exception to the hearsay rule-- it's an admission from opposing party.

Be advised, however, that your employer will not be happy to learn he's been recorded-- if he hasn't already fired you, he will at that point. You certainly have no protected right to record him-- do so at your peril, whether or not it's ultimately admissible in court.

According to Or. Rev. Stat. §§ 165.535, 165.540 your employer must know the conversation is being recorded or it cannot be admissible in court. Also, it is a crime not to inform them with up to one year in jail and a $5,000 fine.