Marine algal toxins of the okadaic acid (OA) group can occur as diol esters and sulfated diol esters in algae and as fatty acid esters in shellfish. Several of these ester forms have been identified, but the most common procedure for detecting OA group toxin esters is by measuring the increase in parent toxin after alkaline hydrolysis. Use of this alkaline hydrolysis method led to the discovery of high levels of conjugates of OA and dinophysistoxins-2 (DTX2) in seawater and of OA, DTX1, and DTX2 in blue mussel hepatopancreas (HP) from Flødevigen, Norway, during a bloom of Dinophysis spp. In the water sample, a C8-diol ester, a C9-diol ester, and a previously undescribed C8-triol ester of OA were characterized using HPLC-MS2, -MS3, and -MS4 in combination with various derivatization procedures. Palmitic acid (16:0) ester derivatives of these diol/triol esters were found in mussel HP and characterized using HPLC-MS2, -MS3, and -MS4. To the authors’ knowledge, hybrid diol-fatty acid esters of OA have not been previously described. Mass spectral analysis showed the presence of two forms of hybrid esters: one with the fatty acid conjugated to the 7-OH of the OA moiety and the other with the fatty acid conjugated to the OH group in the “diol” moiety. In the water sample, the C8-diol ester was the most abundant, whereas in the mussels, the 16:0-C9-diol hybrid ester was most abundant, and only minor amounts of the 16:0-C8-diol hybrid ester were detected, suggesting that C8- and C9-diol esters of OA may be metabolized differently in blue mussels. 7-O-acyl esters of OA, DTX1, and DTX2 are thought to contribute to shellfish toxicity by being hydrolyzed in the human stomach to the parent toxins, and the newly characterized hybrid esters are likely to contribute similarly.