Welcome to the site. Different Expert here. I'm PDtax
The keys to calculating the CSED (collection statute expiration date) are knowing when the tax was assessed (generally April 15th after the tax year; i. e. 4/15/12 starts the statute for a 2011 Form 1040
Add to that the pendancy period of the Offer plus sixty days (if IRS had your Offer 6/1/11, and rejected it 9/1/11, you would add 93 days plus 60 more to the CSED total).
You can compare your math to the IRS transcript, available online, by mail, or if you have a tax pro working with you, through a phone call to the IRS Practitioner Priority Line (1-866-860-4259). That line, exclusively for tax pros, allows your pro to talk with an IRS pro who can read from and fax the transcript out right away. Depending on timing, it may be best to go this way as opposed to waiting weeks for a transcript, even through a 4506-T request.
Because there are other items that can toll, or suspend, the running, the IRS figure may be different that the one you compute. For example, IRS used to be able to ask for CSED waivers (the Forms 870 or 870-AD).
Thanks for asking at Just Answer. Please advise if these is anything specific I can address, such as tax years, type of tax, status of Offer and why it was either treated as rejected (extends statute) or refused (does not extend statute), etc.