Bottom line is that oboists tend to strive for a particular "ideal" sound, and they all tend to shoot for the same timbre (or rather, for one of 3 or 4 timbres, depending on their country of origin). They are generally not used to trying to vary their timbre - they in fact try to avoid it. (This is the diametric opposite of jazz sax players, who tend to strive for an individual and unique sound.) Your options for getting different timbres out of a living oboist are (a) dynamics, and (b) extended techniques (like multiphonics, teeth on the reed, harmonics, etc.).
Oboe and trumpet tend to blend pretty well anyway. If you want them to be closer, mark the oboe up a dynamic level, or mark the part to indicate that you want a rougher sound (not sure what you'll get, though), or mute the trumpets. Or write in a soprano sax instead, or in addition. (I'd suggest a soprano sarrusophone, but it isn't easy to find someone who will play it in public