It's not just adding one more electron. Another proton is also added to the nucleus. The overall trend across a row is driven by shielding effects.

In general, as you go across a row, the shielding effect is fairly small compared to the effect of increasing nuclear charge.

As you note, there is a slight deviation in this trend between 5A (i.e., $\ce{N}$) and 6A (i.e., $\ce{O}$) because the extra electron in oxygen is paired. This increases the electron-electron repulsion, so the ionization energy decreases slightly.

As you note, $\ce{F}$ (7A) resumes the overall trend of increasing ionization energy driven by the attractive effects of the higher effective nuclear charge $Z_{eff}$.

How adding one more electron increases the ionization energy?
When i added service reference to my .Net project (right click->Add Service Reference), VS created a folder with the Service name and created bunch of files (listed below). I am not sure which files should be added to the source control so that next time when i upgrade the service reference, i dont end up deleting some files and adding new files. List of files that VS created in the service folder are as below:

ConfigurationService.disco
ConfigurationService.wsdl
ConfigurationService.xsd
ConfigurationService1.xsd
ConfigurationService2.xsd
ConfigurationService1.wsdl
ConfigurationService3.xsd
ConfigurationService4.xsd
configuration91.svcinfo
configuration.svcinfo
Reference.svcmap
Reference.cs
abc.datasource
def.datasource
ghi.datasource
jkl.datasource
mno.datasource
pqr.datasource

