How did you learn to articulate? When I was first picking up the horn, I was taught "ta ta ta". When I got to college, my instructor beat "ta" out of my head and taught me "toe toe toe", and "low low low" for legato.
Obviously, the difference here is the vowel. /a/ and /o/ are both pronounced in the back of the mouth, but /a/ is lower than /o/. That is, your tongue is farther down in the back of your mouth when you say /a/ than when you say /o/. Try saying both "ta" and "toe" and stick your finger in your mouth and feel where your tongue is and the shape. The tongue kind of curves up in the back when you say /o/.
This seems counter-intuitive. Why would you want your tongue in the way of your airstream? However, the location and shape of the tongue has another effect. Try saying "ta ta ta", and put your hand on your chin. Your jaw moves, doesn't it? Now say "toe toe toe", and feel your chin again -- not so much movement, right? One thing you don't want when you play is a lot of jaw movement -- it moves the teeth, interferes with the placement of your lips, and can generally muck up your embouchure. By using "toe" instead, with less jaw movement, we learn not to move our jaw when we play just by thinking of a different syllable.
Try holding the horn with your left hand and articulating while feeling your jaw with your right hand. Think of saying different vowels when you tongue, and feel what happens.