Imac late 2015 with external ssd.

Bit of a newbie question but cant find it covered anywhere.

I currently boot my iMac from an external ssd drive and just use the original HDD for time machine back ups.

Couple of things

1. Anything i need to consider/do differently when updating my OS with open core?

2. Can i just install the OS straight to the ssd and boot it from that rather than using an external USB flash drive?

Any help would be much appreciated.