What are the odds of ultrasound missing ovary tumor? I'm 20 years old and recently have been feeling bloated and gained a bit of weight, so my doctor had me do a trans vaginal ultrasound and an abdonimal xray. The ultraound didn't show anything abnormal

Possible. No imaging study is 100%. There are many reasons you may be having these symptoms. If your symptoms continue, other causes need to be ruled out so continue to see your doctor.
X-ray. X-ray images are very poor at detecting ovarian tumors; however, a pelvic (transvaginal) ultrasound is extremely good at checking for ovarian tumors. A pelvic ultrasound is the preferred imaging modality to examine for any abnormalities of the ovaries or uterus. If your ultrasound was negative then there is essentially no chance you have an ovarian tumor. A common, frequently benign tumor of the ovary in a young woman is called a teratoma. This tumor frequently does contain calcifications (and occasionally well-formed teeth!) which can be seen with an abdominal x-ray, but ultrasound is usually much more sensitive for detecting these tumors, especially if they contain no calcification.