  Simple closed-box (CB) models of chemical evolution are extended on two
respects: (i) simple closed-(box+reservoir) (CBR) models allowing gas outflow
from the box into the reservoir or gas inflow into the box from the reservoir
with same composition as the preexisting gas and rate proportional to the star
formation rate, and (ii) simple multistage closed-(box+reservoir) (MCBR) models
allowing different stages of evolution characterized by different inflow or
outflow rates. The stellar initial mass function is assumed to be universal,
and mass conservation holds for the whole system (box+reservoir) while it is
violated for each subsystem (box and reservoir). The theoretical differential
oxygen abundance distribution (TDOD) predicted by the model, under the
assumption of instantaneous recycling, is a continuous broken line, where
different slopes are related to different inflow rates. For an application of
the model (a) a fictitious sample is built up from two distinct samples and
taken as representative of the inner Galactic halo, and (b) different
[O/H]-[Fe/H] empirical relations are deduced from five different samples
related to different methods, and two of them are selected for determining the
empirical differential oxygen abundance distribution (EDOD) with regard to the
fictitious sample. In both cases the EDOD is represented, to an acceptable
extent, as a continuous broken line. If the inner halo and the metal-poor bulge
(after the inflow stage) are represented by the box and the reservoir,
respectively, then the inner halo fractional mass (normalized to the halo) is
comparable with, or exceeds by a factor up to 4, the metal-poor bulge
fractional mass (normalized to the bulge), for current estimates of the
halo-to-bulge mass ratio of about 0.05-0.10.
