Scientific belief is an ackward but not invalid phrase. The true oxymoron is scientific proof, for the process of science can incrementally increase our confidence in a theory, but it can never prove it (see Popper, 1934). Thus to describe a current theory as a scientific belief is not untrue.
oxymoron: a figure of speech by which a locution produces an incongruous, seemingly self-contradictory effect, as in "cruel kindness" or "to make haste slowly."