Hi Winnie,
First, you must know that in English, they have some pairs of consonants which contrast.
For example: F/V, P/B, S/Z, T/D.
For the red ones, your vocal cords must vibrate. For the blue ones, they don't.

In Chinese, the difference p/b, t/d, j/q, k/g consists in air flow, while in English, it consists basically in vibration. In Chinese, the vibration of the vocal cords doesn't affect much the pronunciation, but in English, it's of uttermost importance.
Oben (if it existed) would NOT be pronounced the same way open is pronounced.

You can try to practice the difference between each one of this pairs and try to carely observe what the difference really is (note that the position of the vibration will be a little different depending on the consonant). And also while listening to words, try to distinguish the first ones from the second ones!
Pronunciation of OPEN : http://howjsay.com/index.php?word=open&submit=Submit

You can find more information about it on Wikipedia for example, where they explain that and give a few examples.

http://en.wikipedia.org/wiki/Voice_(phonetics)

Hi Winnie.

Its pronounced open.

If you need more help with your speaking, you can book a trial session with me.

@Vitor. Wow! Thanks! And here I am pronouncing it like U for over a year! Haha!

The CHU vs ZHU and all the combinations of ch and zh were difficult for me to pronounce at the beginning. Most of the time all of my CHs sounded like ZH, and my ZH didn't sound Chinese at all! which made diferentiating between these 2 impossible at one point, 知道 vs. 迟到. It really helped that I knew the tones though =-D

It seems that the umlat is not written because there would be too many umlats (in pinyin) when writing qu and ju in pinyin.

Everything in your reply looks correct. You might want to write, "Feel free to correct my English mistakes", but that seems more like a personal prefference in writing rather than a mistake.

加油！

[...] 

PS: In English, they do have something like the Chinese P sound. When the stressed sylable of a word starts with a P for example, the P is pronounced with a strong air flow most of the time (but your vocal cords should still not vibrate!).
Example : PILLOW
http://howjsay.com/index.php?word=pillow&submit=Submit

Hope it was helpful!
I would suggest you to practice these pairs of consonants with a teacher (at least in the beginning), since it's about a subtle difference of pronunciation :)
[readers, feel free to correct my mistakes in English ^^]