OMFR, short for "Oh my fucking rod." The term "rod" is short for Rodney, from the internet video titled "Bubble Tea." This could also be interpretted as a paraody of OMFG, by replacing the g in God with an R.
Stems from the phrase OMG.
Sammy the Cashier: MUahahahahahaha! Muahahahah!
(OMFR sounds like he's dying! Call 991!)