The usual solution is to reduce the enlarger’s light output rather than stop the lens down excessively.

Best option: use a neutral density (ND) filter in the enlarger’s filter drawer or light path. An ND filter cuts light without changing focus behavior, and is the standard way to get longer print times for easier dodging and burning.

Other workable options mentioned:

• Use a lower-wattage enlarger bulb, if your Omega D2 supports it.
• For black-and-white printing only, some people use a lamp dimmer, but only if the enlarger lamp runs directly from line voltage and the lamp is the only thing on that circuit.

In practice, ND filtration is the cleanest and most generally recommended approach. A longer exposure time gives you much better control for darkroom techniques while keeping the lens near its sharp aperture.