I presume by "implicitly" you mean, given the value of the integral, you are to find the necessary upper limit of integration (or lower limit, or both limits dependent on some common parameter.)
If so, just use matlab's 'fzero' function where the function it calls on is the integral value over the variable limit(s) minus the required value. You can find this integral using either the symbolic 'int' or the numerical quadrature function, 'integral'.
Incidentally, you may have picked a poor example. If 'o' really means zero, the integral of 1/sin(x) would be divergent at that lower limit - that is, it would approach infinity as your lower limit approaches zero.
