California Nebula in Ha, [SII] and Hb light

I decided to shoot some Hbeta last night and turned my attention to
NGC1499/Calif Nebula.

It was interesting to me to see no [OIII] at all yet pretty significant
Hbeta emission. From an ionization potential perspective, Hb is greater than
[OIII] but there is little if any [OIII] in that object.

Perhaps there's just not any oxygen in the cloud to get ionized?

Anyway, here is the image:

http://www.rdcrisp.darkhorizons.org/ngc1499_emission_page.htm

This time I assigned Ha to Red, and [SII] to Green with Hb assigned to Blue.

I had a bit of a light gradient that I think originated with a light fog we
had last night. I think that caused reflection of the city lights, despite
the use of emission line filters.