Complex sentence: If neither are available, it is appropriate to use "oe" and "ae", although the use of the digraph "oe" reduces legibility to some extent, especially when the diphtong "øy" is rendered as "oey".
Split into simpler sentences: If neither are available, it is appropriate to use "oe" and "ae". As stated before: this should only be done if neither Ø/Æ nor Ö/Ä are available since the use of e.g. "oe" reduces legibility to some extent, especially when the diphtong "øy" is rendered as "oey".