Been taking 1/4 of a oxycodin extended relese tab daily for a week. Took a fourth this morning about 4-5 hours ago. Is it safe to have one shot of rum?

Yes. ...as long as it is measured in the 'official' way with a shot glass and not creatively poured.
Prescribed medicine. It is usually recommended that oxycontin extended release tabs should not be cut or crushed. Check your prescription and/or check with your doctor. If you only need 1/4 tablet, perhaps you should be taking a different form of this medication or a different medication. Medication should only be taken as prescribed. Alcoholic beverages should not be consumed while taking narcotics (eg. Oxycontin.