Oral fluid has been gaining more acceptance as the alternative matrix for forensic toxicology. Currently, Δ(9)-tetrahydrocannabinol (THC) is used as the primary target for detecting cannabis use in oral fluid. Meanwhile, THC carboxylic acid (THCA) in oral fluid is reported as a more reliable marker for cannabis abuse as its presence does not come from passive exposure. An analytical method for simultaneous quantitation of THC and THCA will be efficient for toxicology laboratories. THCA quantitation is challenging due to its very low concentration in oral fluid. Recently reported liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based methods achieved sufficient sensitivity but involved complex sample preparation procedures. We aimed to develop a sensitive LC-MS/MS method for simultaneous quantitation of THC and THCA in oral fluid with low-flow liquid chromatography and a Q Exactive mass spectrometer, using offline sample preparation of oral fluid followed by microflow LC with online sample cleanup. The total runtime of the method was 12.5 min. The method had a lower limit of quantitation of 7.5 pg/mL and was linear from 7.5 to 300 pg/mL for THCA. The intra- and interbatch precision of the method ranged from 3.3% to 9.3% for THC and THCA.