First of all, make sure that you are not confusing the terms "overblow" and "blow bend", they are very different techniques.
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From the TERMS AND DEFINITIONS page-

"Blow Bend- Blow bends are typically done on holes 8, 9, and 10 blow on diatonics. Holes 8 and 9 will bend down a half-step while hole 10 blow will bend down a whole step. Blow bends should not be confused with "overblows" which is a completely separate technique."
http://www.harmonicalessons.com/members/advtech3.php

"Overblows (overdraws)- An advanced technique used on diatonic harmonicas to get the remaining missing notes from the chromatic scale that a player cannot get by normal blow and draw notes or by standard bending technique. The overblowing and overdrawing will cause the note you are playing to go up in pitch and is done on the reeds that do not normally bend at all (i.e. 1-6 blow and 7-10 draw).

As well as being very difficult to do, this technique can only be done well on certain types of diatonics (the Hohner Golden Melody is the most common choice) and these harmonicas normally must be adjusted to facilitate the overblowing (typically by lowering the reed gapping). These adjustments to the harmonica may create other problems for the average player and especially for players just starting out. It is also very difficult for players of all levels to play overblow and overdraw notes in tune.

Overblows and overdraws are useful for some advanced players, but we don't recommend the technique for beginners or the average player. If you would like to play music that involves a greater use of notes than what is available on a standard diatonic (i.e. jazz, classical, or more sophisticated pop melodies), the best and easiest choice would be the chromatic harmonica."
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Lee Oskars do not "overblow" well, but work great for all typical bends including "blow bends".

Let me know exactly what you are trying to do and I can be of more help.

Play on,
Dave Gage

Am I correct about the following: On a C Harp, Holes 4,5,&6 can give you (with overblows) Eb, F#, Bb? Hole 7 can give you (with overdraw) C#. Holes 9 and 10 also have overdraws giving you Ab and C#.

In other words, an advanced player can use a standard 10 hole diatonic and get a complete scale covering three octaves. How cool that would be. I simply do not know how to do an overblow, and have never seen any explanation or instruction. I can only do overbends on 8, 9, and 10. I use 8 frequently. I found the instruction for overbends on this site.
Thank you for your help!
Montana