Google Sheets, multiple sheets, different rows

Question

First, I apologize if my use of quotations and such are not correct. I will try my best to explain my situation and I appreciate any help provided.

I have one Workbook with multiple sheets.

Sheets are titled, "This Month" "Last Month" and "Compiled Data".

The rows of data on "This Month" and "Last Month!" do not match.

For example on "This Month" the information in Row 26 (1420576) does not appear at all on "Last Month".
Sometimes the reverse is true. The information is on "Last Month" and not on "This Month". For example, Row 68(1608785) is on "Last Month" but not "This Month".

I need to add the information from Column O from "This Month" to the information from Column O on "Last Month" and I want that information to insert into Column H on the "Compiled Data" sheet.

I have tried to use =sum('This Month'!$O$2)+('Last Month'!$O$2) but that doesn't account for the information that is different between the rows from each sheet.

How can I use a formula or something to complete this task without manually comparing each row?

Here is an example sheet to reference.

https://docs.google.com/spreadsheets/d/1cNBYkZbWAkhnqpUfr4Attkyy-ThAylsJFVnP-kSh5F4/edit?usp=sharing

Answer 1

Thank you so much for the response.

Issues For "Last Month"

1. #NA is deliberate. I don't have data to include. Is there a better/different way this should be represented?
2. 0 is the same as 0:00:00. There is no length of time to report. Deliberate (kinda). Should I have it all the same?
3. I deleted the #NA from Row 488. It should not have been there.
4. Rows 487-1000 are blank.

Issues with "This Month" Text values vs numeric values. An error. They are identifying #'s and don't hold any value. How do I correct it?

Again, much appreciated.