Hey @Accountingbeans, I hope this helps and I hope I didnt confuse you more.
The AGE in SIRAGE is always booked to OCI with the offsetting entry to plan assets or PBO (usually PBO since most companies always have obligations). Think of OCI is your purse or wallet, you put things in there and take it out as you need it.
A is the unamortized of prior service service period adjustments. This is an expense is the initial entry is DR OCI and CR FV of assets or PBO.
G is the unamortized gains/losses from changes in actuarial estimates (and the difference between expected return vs actual return on your plan assets when you use expected return in calculating ending FV of plan assets). If it's a gain, you DR FV of assets or PBO and credit OCI. If loss, you DR OCI and CR FV of assets or PBO.
E is the unamortized of Existing net obligation or net assets. This is an expense so the unamortized amount is DR OCI and CR FV of plan assets or PBO.
Now when you calculate the periodic pension plan expense (SIRAGE) the SIR is immediately expensed to the P&L. The AGE is amortized based on rules the book gives. The amortization is done for each A, G, and E. Now the unamortized amounts are sitting in OCI so you need to pull out the periodic amortization so you can expense on the P&L. (The gain will offset expense).
For A (original DR OCI CR PBO), the periodic amortization now DR P&L, CR OCI.
For G (original DR PBO CR OCI), the periodic amortization is now a DR OCI, CR P&L.
For E (original DR OCI CR PBO), the period amortization is now DR P&L, CR OCI.
You're basically reversing the original entry for the periodic amortization amount.
At the end of every period, OCI is always cleared out to AOCI. For example, when you book your initial unamortized A G or E in year 1, it is cleared to AOCI at the end of year 1.
Now when we are in year 5 and amortizing this stuff, the initial unamort is already sitting in AOCI. However, you book the reversing of the original entry for the periodic amortization amount. This will also get cleared out to AOCI. When you net the initial unamortization with the reversing amortization amount, AOCI will decrease (or increase if you have a bunch of gains) until it's fully amortized on the P&L.
.