There's a typo in your question, Oxygen has an Atomic Mass of 15.9994 not 15.9949.
Atomic mass of an element is calculated by taking the atomic mass of isotope #1 times the percentage of that isotope found in the world (called its "abundance") plus the atomic mass of isotope #2 times its abundance plus the atomic mass of isotope #3 times its abundance . . . until all naturally occurring isotopes are covered. So it's a weighted average. Consequently, the sum of all the "abundances" must add up to 100% or 1.00.
So, if we identify isotope #1 as the one with weight 15.9949 and say its abundance is a1 and if we say isotope #2 is the one with weight 16.9993 and say its abundance is a2 then we can say the following:
1) a1 + a2 + 0.00204 = 1.000000 (notice I converted percent to a decimal)
2) 15.9994 = (15.9949)(a1) + (16.9993)(a2) + (17.9992)(0.00204)
Solving equation 1) above for a2, we get a2 = (0.99796)(a1)
Substituting "(0.99796)(a1)" for a2 in equation 2) above, we get:
15.9994 = (15.9949)(a1) + (16.9993)(0.99796)(a1) + (17.9992)(0.00204)
and then we can solve for a1 to get a1 = 0.99755 or 99.755%
Then plugging 0.99755 in for a1 into equation 1), we get a2 = 0.000409 or 0.0409%