On the Use of Field RR Lyrae As Galactic Probes: IV. New Insights Into and Around the Oosterhoff Dichotomy*

We discuss the largest and most homogeneous spectroscopic dataset of field RR Lyrae variables (RRLs) available to date. estimated abundances using both high-resolution low-resolution ({\Delta S} method) spectra for fundamental (RRab) first overtone (RRc) RRLs. The iron 7,941 RRLs were supplemented with similar literature estimates available, ending up 9,015 (6,150 RRab, 2,865 RRc). metallicity distribution shows a mean value <[Fe/H]> = -1.51\pm0.01, {\sigma}(standard deviation)= 0.41 dex long metal-poor tail approaching [Fe/H] -3 sharp metal-rich solar abundance. RRab are more (<[Fe/H]>ab -1.48\pm0.01, {\sigma} dex) than RRc (<[Fe/H]>c -1.58\pm0.01, 0.40 dex). relative fraction in Bailey diagram (visual amplitude vs period) located along short-period (more metal-rich) long-period metal-poor) sequences 80% 20\%, while display an opposite trend, namely 30\% 70\%. found that pulsation period steadily decreases when moving from regime. visual same but amplitudes almost two times sensitive metallicity. also investigated dependence population ratio (Nc/Ntot) on we is complex globular clusters. increases ~0.25 ~0.36 regime, it ~0.18 -1.8 < -0.9 ~0.3