Morse, who was acquired from the Washington Nationals this offseason in a three-way trade, drove in four runs to pace Seattle, which opens the season 2-0 for the second time in three seasons. Michael Saunders added two stolen bases and two runs scored for the Mariners.
"We put up some tough ABs tonight. When you do that, good things will happen," Seattle manager Eric Wedge said.
Hisashi Iwakuma (1-0) went six innings for Seattle, yielding just one run and two hits while striking out seven.
Yoenis Cespedes was the lone bright spot offensively for Oakland when he took a 2-1 offering from Iwakuma out of the park for his first home run of the season. Oakland has managed just one run through two games.
Carter Capps and Oliver Perez combined for three scoreless innings out of the bullpen for Seattle.
The Mariners trailed 1-0 after the Cespedes bomb but they quickly responded in the top of the third. After Saunders and Seager drew walks from A's starter Jarrod Parker (0-1), Morse went opposite field for his first homer of the night, giving Seattle a 3-1 advantage.
Parker was disappointing in his 2013 season debut, allowing four earned runs and five hits over five innings. He also walked three hitters and struck one just one.
"I never really settled in, didn't really get comfortable," Parker said "You're not going to be very successful when that happens."
Seattle added to its lead in the fifth inning when Kendrys Morales, another offseason addition, smacked an RBI single up the middle to plate Seager.
After Jerry Blevins and Pat Neshek combined for a scoreless sixth, Ryan Cook took the hill in the seventh inning for Oakland and struggled with his command, walking three batters and hitting another. His bases loaded walk with Justin Smoak at the plate cost his team another run. Evan Scribner entered and weathered the storm by getting Jesus Montero to pop up to end the innings and prevent any more damage.
Seager padded the lead with a two-out bloop single to left in the eighth inning, scoring Saunders who stole second on the previous pitch.
Six of the Mariners' seven runs tonight came with two outs ... Seattle hit just three-for-14 with runners in scoring position ... Franklin Gutierrez roped a double in his only at-bat ... The Athletics mustered three hits tonight, giving them just six for the season.
04/03 02:49:47 ET