I'm going to assume you're talking mtf HRT. It's unlikely that you'll no longer be able to orgasm, but your desires might change. In my experience, testosterone can compel you to more stereotypically male acts sexually, even if those actively cause dissociation. That compulsion will likely go away on HRT, meaning that you will need to find new, feminine acts to achieve sexual gratification. If you find ways to do so that minimize dysphoria, you'll probably be able to orgasm, but if you stick to past techniques you might find that without the T compulsion dysphoria is too great to orgasm. Just experiment as your body changes. Oh, and it might take longer. Finally, taking progesterone can - anecdotally - increase your libido. If you are really concerned, try simulating a menstrual cycle with your hormones. You'll likely find that some point in your cycle significantly increases your libido.