That's great news, sandstranger!
I don't know about optimizing and compiling software for Android. Dalvik, ART, and a variety of system-on-a-chip (SoC) types complicate that. People over at
the XDA-Developers Forum may help you if no one here does. If you're building with the GNU Compiler Collection, reading
the online manual for the version you're using could help too.
If your Android build environment is anything like the desktop Linux one, this should work for flags that are common to both C and C++:
~/git/openmw/build$ export CFLAGS="
your chosen flags go here"; export CXXFLAGS="$CFLAGS"; cmake ../ && make
I recommend not using -m (machine-specific) flags until you learn more about what works across different SoCs, or produce different builds for specific types. Bear in mind that performance could always be unsatisfactory because compiler optimizations can't make up for hardware limitations and current deficiencies in OGRE's utilization of GPU features. For your own personal builds,
-march=native -O2 are good to start with. If those don't obviously break anything, you could try
-march=native -O3 next.
Hope that helps