You need the molecular weight of the oligo to calculate that. It should be printed on the tube somewhere. You can approximate by multiplying the length of the oligo by the average MW of a DNA base, which is 330 g/mole for single strand DNA as in oligos.
For your typical 20 base primer, its molecular weight will be around 6600g/mol, which is the same as 6600 pg/pmol. Since pico = 10^-12 and nano = 10^-9, there are 1000 pg in 1 ng. So 6600 pg/pmol = 6.6 ng/pmol. With this conversion it’s thus fairly simple. 1 pmol/ul = 6.6 ng/ul.
The simple answer is to divide the molecular weight of your oligo by 1000 to get the conversion factor, then multiply your pmol/ul by this factor
pmol/ul x MW (oligo in g/mol or pg/pmol)
………… —————————————-… = ng/uL.
(sorry about the dots, it’s the only way to keep the internet from eating the formatting)