Here's my understanding, but you should talk to a lawyer who practices in Texas:
If your entity is "transacting business" in Texas, you are required to file an application for registration with the Texas Secretary of State. They don't define "transacting business," but flipping houses almost certainly qualifies.
You have 3 options:
1. Register your Oklahoma LLC with the Texas SoS. This is the simplest option as far as paperwork, but the registration fee is $750. You still need to file your annual reports with the Oklahoma SoS, as well as complying with Texas requirements.
2. Form a new LLC in Texas. This is the cheapest option with respect to filing fees. $300 filing fee.
3. Convert your entity from an Oklahoma entity to a Texas entity. This gives you the advantage of not having to file annual reports in Oklahoma anymore. It will cost you two $300 filing fees and requires a bit of paperwork. The Texas Business Organizations Code Section 10.103 explains how to do a plan of conversion.