The answer to the question you pose has a lot of depends:
1. Do you have an attorney in mind that you want to use already, if so then that may be a reason to have an OC attorney do it in that you already know the attorney and do not have to do additional homework.
2. What is the mental status of your mother? Most reputable attorneys will not do any legal work in this field without meeting the client and making sure for themselves that the person making the trust (settlor) has not mental status issues.
3. Are there issues with budgeting for this matter? An OC attorney whom you know may be ideal if the budget is there to pay him/her to travel to Temecula to meet with you mother and then draw up the trust. If not then it is probably best for you to go to Temecula and interview attorneys there. Assuming your time is less expensive than the attorney's.
4. Finally, your mother has to be comfortable with the person she works with so that is a big part of it as well.
First question: Is your mother competent to sign estate planning documents?
Second question: Is she willing to meet in OC with the attorney?
Third question: Are there estate planning documents that already exist?
Fourth question: Does she plan to change how her estate is to be distributed (e.g. favoring certain siblings over others)?
The concern for the attorney is whether your mother is capable of signing documents and that the wishes she expresses are her own independent wishes, without undue influence.
Where the attorney is shouldn't matter in that case.
The simple answer to your question is yes you can get an OC lawyer to do the trust for her. In fact you could get any lawyer in the State of California to do the trust for her (for obvious reasons you want one close to you)
Since your on AVVO.com, go ahead and type trust lawyer on the search page and then type in the city that is convenient for you. I am sure you will find a great one. Also a yellow page internet search is great as well.
In my experience, when it comes to trusts, cheaper is not always better. Find an attorney that specifically does trust, granted it might cost a little more but it will be worth it when your loved one passes. The problem with trusts is that you wont know that the trust is bad or good till it is to late, so find a good attorney. Best of luck.
Generally there is nothing wrong with you assisting your mother with finding an estate planning attorney in Orange County.
My only concern would be that if your mother intends to have a trust that is either contrary to a prior estate plan or otherwise doesn't evenly divide the assets between the children. Under such scenarios her trust might be challenged and the level of your involvement in finding the attorney, taking her to the attorney, discussing the estate plan with your mother in advance of signing, or things such as attending the meeting with the attorney or having any contact with the attorney, could be used in an attempt to claim that your actions constituted undue influence in an action to invalidate the trust.
However, if your mom is naming each of her children equally and isn't married and isn't creating an estate plan greatly different from a prior estate plan, then by all means help her out by finding her a quality local attorney.